I like all the ones I’ve joined, naturally - even the ones I only joined to add others’ observations to - but my favorites are probably:
Weird Wild Wonders: I’ve learned so much that helps with unknowns and even helped ID some things here.
Tilly Bones: I got to witness the birth of this project, as spawned from several observations in Weird Wild Wonders.
Nature Drawing & Journaling: An interesting look into alternative ways of documenting encounters for iNat.
Audio Observations from Around the World: technically, I think this is a collection project, but as with Nature Drawing & Journaling, it’s interesting to see other forms of evidence.
Microscopy: Cool pics, and maybe someday I’ll find my old childhood microscope & post something in here too.
Pollution: Not a fun one, per se, but very useful.
And of course these were especially helpful during lockdown: