I enjoy original RPs and fandom-related RPs.  Even if I haven't heard of the fandom I'll occasionally join a RP if it looks interesting, then binge-watch or read the thing it's based off of and research it along the way.  As to some of the more mainstream things I like, here you go:
RWBY
Red vs Blue
Harry Potter
Marvel
Percy Jackson/Heroes of Olympus
LOTR
Hunger Games
Divergent
Bioshock
Portal
Assassin's Creed
Dishonored
CreepyPasta
Avatar: The Last Airbender
StarCraft
Borderlands
Vocaloid
The Evillious Chronicles
~

These are in no particular order, and just what I thought of off the top of my head.
I actually enjoy any and all of those categories, though I particularly enjoy Sci-Fi, Fantasy, and Steam/Cyberpunk RP.