While the game wiki documents each DLC's features exhaustively and answers to the question "Which DLC should I buy for CK2?" can be found all over the internet in countless forums' threads, at least a few diagrams and probably many other materials made by the community, the question still arises surprisingly often whenever the game goes on sale.
In this guide I attempt to provide a view that is more nuanced and player-oriented than the highly-objective information on the wiki, (hopefully) easier to find and more up-to-date than most forum posts out there but still text-based, for those who want something more in-depth than an image guide but don't have the patience for the video format.
The first thing that should be noted is that only about a dozen items in CK2's DLC page actually add to the gameplay, those are the focus of this guide. The remaining titles provide only additional graphics, music or some other kind of content and will be dealt with only in passing.

My method relies on classifying each DLC feature as belonging to one of two scopes:

• Local: this applies to features that unlock or enrich some group of playable characters, such as muslims or nomads, and have no effect on everybody else.
  
• Global: this applies to features that are either available to all characters or that I find reasonable that a player, whatever start he chooses, will eventually be able to use, such as actions only available to kings(my condolences if you like to play as a count for the whole campaign length)

While Paradox never explicitly endorsed this model, I find it describes CK2 and its extra content fairly well, with most DLCs containing almost exclusively features localized to a particular group of characters, and a few containing mostly global features and little to no localized ones. Because of that, the general recommendation I give is: get most or all of the predominantly global DLCs and only the local ones that deal with character types you'd like to play.

One thing many people don't know is that when playing multiplayer you have access to all DLCs the host has, even if you don't own them yourself. That means if you already own the base game and you're not sure about a particular DLC you can just join a multiplayer session hosted by someone who owns it and see for yourself if you like it.

This is actually a bundle of the base game plus 5 DLC at a discount price. It's right here at the top because I find it offers great value for anyone looking to get into the game. The contained DLC are:

• Ruler Designer: to customize your starting ruler's appearance, coat of arms and traits.
  
• Legacy of Rome: greatly expands the Byzantine Empire mechanics and enables retinues for all rulers.
  
• Sword of Islam: lets you play as muslims.
  
• The Old Gods: lets you play as pagans, unlocks a new start date(867) and a few more goodies.
  
• Way of Life: lots of events to improve roleplaying and keep you entertained during peacetime.

You can read more about each DLC in the guide below.

This DLC basically unlocks playing as a muslim ruler and is thus completely local in scope. The additional features available to muslim rulers include the iqta government form, polygyny, decadence and access to special war justifications that make for a very aggressive playstyle. All those mechanics are still used by the AI even if you don't own the DLC.
Without it even if you become muslim through cheating or by abusing mechanics you'll get a game over.

As with all exclusively local DLCs it's up to the reader to decide if the particular characters it affects are of their interest.

In the local scope this is a DLC about the Roman Empire and what remained of it in the middle ages, the Byzantine Empire. Byzantine characters can now castrate or blind prisoners and eventually, if you castrate enough rivals play well enough, restore the Roman Empire to its former glory and mend the schism in the christian faith.

The one chief global feature of this DLC are retinues. Retinues are elite troops, stronger and much more expensive both to obtain and maintain than regular levies. Their better stats, along with the flexibility in army composition they enable make them a must-have in the eyes of many players.

Recommended even if the reader has no interest in the roman/byzantine empire, retinues are just that much useful.

Note: I don't own this one myself and many players aren't very fond of it either.

This DLC creates a fictional aztec invasion of Europe in the late game. Only recommended if you don't mind the ahistorical premise and feel like the game is too easy without hordes of aztec warriors rampaging through western Europe.

Another entirely local DLC. This one lets you play as merchant republics.
Republics are ruled from cities, rather than castles, giving them less troops but much more income, and thus making them more reliant on mercenaries for warfare. By building and controlling trade posts in coastal provinces republican rulers can increase their income and expand their territory by seizing cities and then whole provinces. They also have a special succession law based partly on heredity, partly on diplomacy and intrigue and partly on blowing vast sums of money on electoral campaigns.

As with all entirely local DLCs, I leave the conclusion up to the reader.

With this one you can now play as pagans(nordic, tengri, african, etc.) as well as zoroastrians. Among other things pagans are able to organize raiding parties and plunder other rulers' territories for gold, glory and prisoners. By controlling enough holy sites you can also reform the faith, turning it into an organized religion capable of having holy orders and calling great holy wars.
Both pagans and zoroastrians are able to take concubines as a middle-of-the-road solution between polygyny and cheating.

• A new start date in 867.
  
• Updated revolt system, with more types of revolts available, always having a leader and playing like a war, with the possibility of breaking off as an independent state if not handled properly.
  
• Landless adventurers may take refuge in other courts, amassing troops and eventually launching an invasion for a title they have a claim on, which might very well be yours.

A solid package, recommended even if your interest in pagans and zoroastrians is not particularly big. The new starting scenario is very solid and the new adventurer and revolt systems make the game much more thrilling, if a bit more difficult too.

Basically a flavour pack for the abrahamic(jewish, christian, muslim) faiths in the game. Adds a lot of new events and some new mechanics:

• New papal mechanics for catholics, with a college of cardinals to elect a pope and new interactions.
  
• New holy orders for each religion.
  
• Ability to play as a jewish character.

While entirely local, this expansion affects so many characters in a meaningful way I strongly recommend it unless you have absolutely no interest in abrahamic faiths.

This one lets you play as a ruler of a dharmic(buddhist, jainist or hindu) religion. Indian religions tolerate each other slightly better than foreign religions, and a character from one of them can freely convert to another for a considerable prestige fee. Each dharmic faith also has a few branches(such as mahayana and theravada buddhism), giving small bonuses to its followers but also a minor relations penalty to characters of different branches; Changing branches is also possible by paying a small prestige fee.
But not all is peace and understanding, indian characters also have access to the subjugation casus belli, which allows you to take over a title held by a character of the same culture group as yours if you win the war.
The caste system is also featured, strongly restricting marriage and landing choices for hindu characters but affecting other religions way less strongly.

The sole global feature here is a couple of decisions to convert to whatever religion and culture is present in your capital.

While the conversion decisions enable some pretty interesting cheese plays, it's no must-have. Get it if the local features interest you.

• Lets you play zunist rulers. Zunism is a pagan religion with some special features that only appears in the new start date introduced by this DLC.
  
• Events to simulate the rise and conquests of Charlemagne.

• New start date in 769.
  
• Ability to create custom duchies, kingdoms or empires if you hold enough land.
  
• Emperors can grant Viceroyalties as a way to distribute titles without really parting with them forever.
  
• Dynasty chronicles recording all the achievements of your characters over the centuries.

I'm not particularly a fan of the new start date, but the custom titles creation is nice and viceroyalties are a useful tool for any emperor. The chronicles are a nice idea but in practice they are quite bland and uninteresting.
Overall a mixed-bag, recommended mostly to those with interest in the local features or in playing as an emperor.

A wholly global DLC. It lets characters set a lifestyle focus, unlocking special actions, such as the ability to seduce other characters, throw mad parties and duel rivals, and includes special events which might give you unique traits depending on your current focus.

A must-have, the new events and actions enrich the gameplay significantly no matter where you choose to start, opening new options for influencing other characters and improving your attributes, the new events also help keeping you entertained during periods where nothing of note is happening in the realm.
Some players don't like how often the AI picks the seduction focus, turning the game into giant orgy, I myself don't find it that bad, it has been nerfed a few times already and can be further tweaked with mods and game rules anyway.

• Grants nomads a new government type based on population rather than holdings and clans rather than regular vassals, along with a new succession type. Without it nomads can still be played(as long as you meet all other DLC requirements), but they use regular tribal government mechanics instead.
  
• Silk Road: a new mechanic to simulate the historical silk road trade route between Europe and China. Rulers who own a province in the silk road can now build trade posts in it.

• New mercenary bands can be formed and evolve dynamically, they will offer their services to rulers and grow in size as they earn money.
  
• Rulers can now subjugate other rulers and make them tributaries.

Not too important if you don't like playing as nomads, but still has its value: tributaries are useful to get extra cash out of people you might not want to annex yourself; the silk road, as long as it is open and running, also provides a solid boost if you happen to own a trade post along it; and last but not least the new mercenary system means that it doesn't matter if you're a count looking to hire 500 troops or a king looking for 5000, you'll almost always find the right mercenary band for your needs for the right price.

Another completely global expansion. The chief feature here is an overhaul of the council and law systems for all governments, making both members of the council and vassals(and they really want to be part of the council too now) have more say in the matters of the realm.
To complement that there's a new favors system, which lets you influence(or be influenced by) other characters, forcing them to vote as you do on the council, press claims, join or leave a faction, and more.
And last, Conclave offers a revisited education system, consisting of two phases where in each, along with a guardian, the player may choose one from a number of focuses that will affect the child's final stats and traits.

A controversial DLC:

• The new council mechanics make vassal management significantly tougher, while also giving the player some new avenues to pressure his liege(only if you're playing as a vassal, of course)
  
• The new education system demands more active management from the part of the player, while also making it somewhat unreliable to mass-produce perfect children just by picking a god-tier guardian like before. The latter makes it particularly unpopular among min-maxers.

All in all not a bad DLC, but it adds substantial complexity and micromanagement to the game and thus might be undesirable to new players.

Another almost completely global expansion. The chief feature this time is a big overhaul to all aspects involving health: diseases have been fleshed out, causing a series of symptoms to appear before a diagnosis makes their nature fully clear, court physicians can be appointed to treat them in a variety of ways, sometimes with gruesome results, epidemics(including the black death) have been reworked and new ways to deal with them have been added too.
Prisoners can also be dealt with in more ways, such as humiliation and torture, and the already existing ways were enriched with more flavour, like new texts detailing the exact means by which your prisoners were executed.
Last but not least there's the prosperity system: just like provinces can now be ravaged by disease and destruction, in the lack of those they can thrive and enjoy benefits such as greater wealth and lower revolt risk. You can also pick one province to be a crown focus, netting a boost to the time it takes to prosper.
Oh, and cats. There are pet cats now.

While the new disease/epidemics system makes the game considerably harder, often ending characters' lives way before(and in far more uncomfortable conditions) what used to be the norm, the new prisoner interactions are a great addition to those of a more sadistic inclination and the new prosperity system has some nice events to keep you busy during peace time, also giving some nice bonuses once you get your provinces to start prospering.

The big feature of this expansion are societies. By joining one your character can receive missions which grant favour with them once completed, which in turn can be used to perform special actions or climb in rank.
Different societies will be avaiblable to different religions, most of them having at least one available. The types of society unlocked by this DLC are:

• Monastic Orders: lets you become a monk and do things like taking vows of celibacy, pray/meditate and perform charity.
  
• Hermetic Society: you can build a laboratory, do research in alchemy and astrology, raid other characters laboratories, cure stress and depression with potions, go search for ancient texts or ingredients and more.
  
• Devil Worship: lets you summon familiars, curse other characters, desecrate temples, sacrifice prisoners, possess other characters and much(really much!) more.
  
• Hashashin: available only to Shia muslims, this society revolves around intrigue and murder, making it easier for you to conduct assassination plots, letting you abduct and intimidate other characters and even letting you raise an assassin army and fleet. It even lets you take hashish!

Do note that the society system itself is not part of the DLC, so you don't necessarily need it to play mods that use custom societies.

The expansion also contains various event chains that let you embark in adventures in search for relics or hire craftsmen to make you fine weapons and jewellery, granting you various bonuses.
Those items are kept in your treasury, which, much like the societies system, is a part of the free patch, not of the expansion itself.

Apart from those most other features are oriented towards quality of life improvements:

• Councillors now have one new job each.
  
• You can now give orders to allied armies in wars(provided you're the war leader, of course).
  
• The ability to perform some actions in bulk on prisoners, like ransoming or executing everybody.
  
• Buttons to suggest names for newborns, like random(culture-appropriated) name, name after parent and so forth.

This DLC also contains two sets of portrait and unit packs, for both english and german characters.

The bundled societies are a mixed bag. Some, like hermetics and assassins, are extremely powerful, others, like most monastic orders, feel rather bland and generic. Another thing to keep in mind is that a lot of characters, mostly pagans, don't have any society available except maybe for the occasional devil cult.
What really drives the value of the DLC past the "nice gimmick" level for me are the QoL features. Ordering your dumb allies around makes for a really satisfying warfare experience and the bulk prisoner actions are wonderful to keep your dungeons tidy and clean after a successful siege. The absolutely gorgeous portrait packs are just the icing on the cake.

All in all a very nice to have expansion, but of limited value for people playing religions without societies, who don't want to bother with giving allies orders or don't care about those horrible portraits central europeans and englishmen had in the stock game.

I don't own this one yet! Read about it on the wiki and make up your own mind.

I'm still evaluating this one(it's huge!) but so far I'd say it's well worth the price.

The text above covers all major DLC, the ones that add/unlock features to the game. Below I'll talk about the remaining ones, prioritizing two classes of DLC that are of particular value: cosmetic and customization DLCs.

Those come in many flavors, I particularly recommend getting at least the face and clothing packs, and if you happen to be a heraldry fan, the shield(coat of arms) packs:

• Character portraits are something you'll be looking at all the time, and having more variety and accuracy in their presentation can really enrich the game experience.
  
• The coat of arms packs contain elaborate, but not always quite historically accurate, coats of arms for many prominent dynasties in the time span covered by the game. If you know enough about heraldry to be bothered about the simplicity and inaccuracies in the base game(I don't!) then you'll probably want to take a look at those.

All the other graphical DLC add new 3D models in the game map. Most of the time you'll be too zoomed out to apreciate them in all their detail, but they might interest you nevertheless:

• Unit/Model packs add new graphics for armies, boats and councillors.
  
• The one sole Building Pack for the game adds some famous landmarks to the map, such as Hadrian's Wall and the Pyramids.

• The customization pack[www.ckiiwiki.com]: this one lets you customize your ruler's appearance and name your grandchildren and beyond, instead of just your children, as well as your titles.
  
• Ruler designer[www.ckiiwiki.com]: lets you pick a character on the map, tweak him around and then play as him. It has some restrictive rules to prevent you from creating the god emperor of mankind, but is still useful for roleplaying purposes.

Those are self-explanatory. Most of the tracks are orchestral and evocative of a particular region and/or culture, they are also set up to be more likely to play when you're playing a character of that type.
Then there are also two that have "metal" in their names, those contain, you guessed it, metal music. A treat for metal fans, but potentially immersion-breaking for those who are not into it.
Speaking of immersion, in the "Orchestral House Lords" pack the composer decided to experiment with electronic remixes of some songs of the base soundtrack, which some also find rather unappealing.

The above covers most relevant extra DLC. Before going further I'd like to talk a bit on how they are usually packaged, since not all of them are available for purchase individually:

• Individually: most music packs, and most graphical packs released before Horse Lords can still be obtained by themselves. I won't list them here, for they are too many.
  
• Ultimate packs: those packs bundle together all extra DLC of a certain kind that was previously only acquirable individually. Also a bit of a misnomer since, for instance, the Ultimate Portrait Pack doesn't contain the portrait packs bundled with major DLC or in Content Packs. Other packs like this are: Ultimate Unit Pack, Dynasty Shield Pack and Ultimate Music Pack
  
• Bundled in major DLC: some portrait and unit packs are bundled inside a major DLC, usually related to the content in it. For example: Rajas of India came with bundled unit and portrait packs for indian characters.
  
• Content Packs: some of the major DLC had a content pack associated with them, containing portraits, units, sometimes music, and other additional content in a single package. The three that exist are the content packs for: Horse Lords, Conclave and Reaper's Due

• CK2 to EU4 converter[www.ckiiwiki.com]: if you also own Europa Universalis IV you can use this to convert your CK2 savegame to EU4 and keep playing there.
  
• Ebooks: some medieval historical fiction pieces. They don't really have anything to do with the game and are only here for completeness.

If you have read this until here, then congratulations, you now know what each of the more than 50 items in the CK2 DLC list do!

I'd like to thank all the contributors to the game's wiki[www.ckiiwiki.com] for extensively documenting each patch and DLC's features and keeping it reasonably up-to-date and Paradox, for making the game and(hopefully) not coming after me for defacing their beautiful art for Way of Life.