Draw 2 more cards every turn.

This is useful for every build, basically. Unless you're playing with a tiny deck (which is very difficult to do on NSI), then extra cards is always good.
Especially good/risky for some Armament runs as curse cards can clutter a small hand size, but it means they might take effect more often.
Probably the absolute best style for this is anything that uses low cost/free cards. Green Aspect, for example.

Increase max HP by +10,000.

Always useful. 10,000 is a lot (worth 5 of the little fountain nodes), so I generally suggest ignoring them and beelining here if that's what you need for your build.
Every build in the game can use this, obviously, and even ones like Phase 2 Only runs can use this to increase the max health of the Phase 2 stage.

Lava tiles deal 100 damage to all enemies at the end of the turn.

Quite useful against hordes like hero-less armies, or towns, but drops off in potential against single bosses. The best way to use this is a full-fire build (which is, frankly, almost vital to even get this relic), as that makes it easiest to cover the whole field in lava.
Beware: Some bosses will use this against you. I believe it's Master of Hammers who gains armour for lava (offset a bit by this relic), but Dorian will massively buff himself AND remove all your lava.

Side note: I really dislike this mini boss. He has so much health that even when the field is mostly covered in lava, he takes a year and a day to die.

Meteors deal +1500 more damage.

This is probably the mini boss relic most specific to a certain playstyle. As it boosts meteors (which land at random), you should really pair it with Boss Mechanics and other stuff that maximise the number of meteors you cast.
There are numerous ways to have a veritable barrage of meteors, so this can be very potent. Just be aware that their random landing patterns can mess up other combos such as tornados and armour generation. That's why I only use meteors when I'm going full fire.

In addition, at time of writing, there is a bug with Molthrax so his rewards appear twice after defeating him. You can still only claim 2 things (there are 2 max health and 2 relics), but it's possible to claim the eye twice. I don't know if this gives meteors a total of +3000, but I assume it does.
I expect this will be patched at some point.

Everytime a Whelp dies, spawns 2 Firebombs.

This one is less universal than most of the others, but I've used Whelps in a variety of playstyles. Whelps tend to die a lot, and paired with the Mechanics and cards that spawn a lot of them, this can be a good turnover of Firebombs too. Pairing this with relics that increase Firebomb damage would make things a lot better.
Just be aware that Firebombs will spawn at random, so terrain-heavy playstyles may struggle a little to make use of them, but at least they don't break combos by triggering a series of terrain changes.