I have full combo on almost every hard chart in the game and I've played for quite a while, so here's my take on things.
Note that this is aimed for people who can already read 9 charts properly and who want to improve and be able to play the most difficult charts in the game
Of course this will help you get a couple achievements like rhythm game master quite easily.

I'll go chapter by chapter, and I'll only talk about 9 and 10 difficulty charts, if there are charts I do not mention, then it means that they're simply not that hard and can be played just fine if you manage 8 difficulty
I'll try and rate the songs like this : Lowe 10, middle 10, higher 10 and extreme 10 difficulty. Lower 10 being a difficult 9 or easy 10, and extreme being comparable to Stone Monument songs.

*Deemo's collection 1

- Entrance
a bit hard but on the easy end of 10 difficulty, great song to learn with because it's easy to read the patterns

- Magnolia
About the same as Entrance, but it's longer so you'll need more stamina, an other great song to learn with, particularly because it has patterns you will be using a lot

- Utopiosphere
Great chart for learning, it's fast but easy to read, and easier than other difficult charts

*Vocal collection

- Ukakuf kins
It's actually a tricky chart, harder than it looks

- Re:you
About the same difficulty as Ukakuf kins

- Chance
One of the most difficult chart in the game, needs crazy high apm

- My dear, Deemo
difficulty is okay once you can play that one pattern it has a lot of, you can use it to learn this pattern, but it's not very efficient, lots of downtime and long chart

*Deemo collection 2

- Myosotis
Weirdly the only 11 difficulty in the game, yet it's far from being the most difficult chart, it's hard like a middle 10

- Anima
funnily right next to Myosotis, and harder than it, I consider it a higher 10, but it has one pattern that is really hard, so really difficult to full combo

*Golden sheets

- Aitai
It's very similar to Anima, just a tiny bit easier and less interesting

*Reborn collection

- Blue evenfall
useful song to learn a certain pattern, beside that, it's not hard if you can manage repeating notes

- Dawn of the ruins
overall not that hard, but it has a couple tricky parts, and it's really long, good to train stamina and burst of speed

- Accelerando
higher 10, quite unique in its patterns, one of the best song to train separating hands and hands coordination

- Tempestuous
middle 10, only the beginning and end are difficult

- Squeaky mind
Weird chart, lower 10

- Place your bets
Play it a lot, really great great song to train, difficulty-wise it's a lower 10, quite technical

* Mili collection

as much as I love Mili, those songs aren't particularly difficult, they are a bit long however so can be used to train stamina to some extent

*Eshen Chen

there's only 5 and 6 difficulty here...

*Aioi

again, nothing difficult here

*Rayark selection 1

hum yea....

*Rayark selection 2

- Altale
Lower to Middle 10, good song if you need to train "rain" patterns, they're quite fast in this song

- Veritas
Higher 10, borderline extreme 10. Really fast, you need to be very precise, easy to read, hard to play

*NMST

not much there

*Knight Iris

- Forbidden codex
extreme 10, pretty sure it's one of the fastest stair pattern chart in the game

*Knight Rosabell

- Predawn
higher 10, good to train a certain pattern you will need for a few extreme charts

- Where you are not
middle 10, not much to say about it

*M2U X Nicode

- Lune
higher 10, quite good to train on a few patterns that are used in many charts, it also has one unique group of notes that only appears in this chart

- Wicked Fate
definitely an extreme song, for a long time I couldn't even read it, don't play it if you aren't good enough to understand it, otherwise it's frustrating

*Greatest hits 1

- Revival
One of my favorite song and chart, definitely an higher 10, it's amazing to learn certain advanced patterns, I suggest you learn the last section's patterns by playing other charts like Forbidden Codex

- Oceanus
middle 10, I haven't played it much though

- Lili
I love this one, it's a lower to middle 10, actually good if you struggle with revival's patterns as it has similar ones, also good to learn some patterns you need in many charts

*Greatest hits 2

- Angelfalse
lower 10, nothing to say

- Infinite Puzzle
not particularly hard but I'm including it because it could be use to learn 2 patterns that are quite useful

*Taiko collection

- Rin
higher 10, it's quite varied in patterns so it can feel a bit harder than songs on the same difficulty range


And now, The stone Monument Songs

there's 5 of them, they are the 5 most difficult songs in the game, 4 of them test your skills in one or two areas in particular, and they have the most difficult patterns in that area, how difficult they are will depend on what kind of patterns you are best at
Then there's etude, it's overall the most difficult chart in the game, it has a bit of everything, is extremely fast, has a ton of notes, and is highly technical to play. Good Luck

Remember to warm up on easier songs before playing difficult ones. If you want to improve fast you should play a mix of songs that you can play well (90%+) and songs that are a bit too hard for you without being way too hard (songs that you can reach 80% on, 75% is fine for stone monument level charts) and try to only play songs that you can read properly, you should be able to read 90-95% of a chart in order to learn how to play it. And I suggest that you make yourself some objectives, some you can reach and others that are too hard for now, some short term and some long term, it'll help you a lot in pushing forward.

Hope this will help some people.