Most popular RPG: D&D 5e
Second most popular RPG: Old versions of D&D
Popular RPGs that aren't D&D:
Pathfinder 2e
Daggerheart
Shadowdark
Mork Borg (and variants, e.g. Pirate Borg)
Draw Steel
Call of Cthulhu (especially in Japan)
There's lots more, obviously. I'm sure you'll get dozens more names. But that's my short list. I think it covers a lot of what's "popular".
If you're looking for anything in particular, it will help to give more guidance than just "popular". There's so much niche content out there. Old School Renaissance stuff, story based stuff, mini based stuff, VTT based stuff. What do you like?