Pauper is my favorite, so much room to brew and such a wide open metagame. Even though there's a huge card pool, new sets still often shift the meta enough to keep things fresh, without totally nuking the format like Modern.

It's a powerful format but without a lot of individually powerful cards, so you see a lot of unusual and kind of janky synergies you don't really see anywhere else.

A competitive deck costs as much as one shockland so you can put something together on a whim and experiment.

I mostly play dimir terror, but also have pestilence, elves, and orzhov blade decks I'm kind of iterating on.

Pioneer is also pretty good. And I play commander, like everyone, but I've been less interested in it lately.