Depends if you're willing to play games with dated graphics. Some of the best games are older or had small budgets. These are off the top of my head.
RPGs:
Definately check out The Witcher. Otherwise:
Mass Effect 1 & 2, Star Wars: KoToR 1 & 2, Neverwinter Nights (old), Neverwinter Nights 2 (not so old), Baldurs Gate 1 & 2 (very old), Icewind Dale (very old), Planescape Torment (old), Mount & Blade (low budget).
It's also worth checking out Deus Ex (old), which in my opinion is the most rewarding single player game ever created and mixed FPS and RPG genres together.
Also, if you haven't bought them on PS3 yet, it's worth getting Oblivion, Dragon Age: Origins and Fallout 3 as the modding community on those 3 games is amazing.
FPS:
Metro 2033, Half-Life 2 Episodes (check out Source mods too), Left 4 Dead 1 & 2, Stalker series, Counterstrike: Source, Crysis.
RTS:
Starcraft 2 is the obvious one, but also don't forget about the original which is still great. The Total War series is also worth looking at and is fairly unique. I've always been a bigger fan of Age of Empires. Many will tell you 2 was the best but 3 and Age of Mythology are worth a look at too and have decent campaign modes. If you do like Age of Empires then you'd probably also like Rise of Nations. Warcraft III was released before World of Warcraft and is a nice look into the Warcraft universe without having to sign up to the MMO. Finally, Darwinia is a good little indie title.
