These are my favorite games on the platform, and all of them are really high quality. And yes, Earth Defense Force is a budget game but I had a lot of fun with it.
RPG's:
Mass Effect, Tales of Vesperia, Blue Dragon, Lost Odyssey, Culdcept Saga (not really an RPG, but close enough)
FPS/TPS:
Bioshock, Dead Space, Resident Evil 5, The Orange Box, Left 4 Dead 2, Call of Duty 4/MW2, Earth Defense Force 2017, Rainbow Six: Vegas
Fighting games:
Street Fighter 4, BlazBlue, Tekken 6, Super Street Fighter 2 Turbo HD Remix, Virtua Fighter 5, Marvel vs Capcom 2
Action games:
Ninja Gaiden 2, Devil May Cry 4
Open world:
Red Faction: Guerrilla, Crackdown, Bully: Scholarship Edition
Racing games:
Project Gotham Racing 4, Burnout Paradise, Forza 3
Don't know what to classify these games as:
Banjo-Kazooie: Nuts and Bolts, Dead Rising, Batman, Condemned: Criminal Origins
XBLA:
Bionic Commando: Rearmed, Braid, Castle Crashers, Geometry Wars 1 & 2, Ikaruga, Lumines Live, Mega Man 9, Pac-Man: Championship Edition, Rez HD, Shadow Complex, Super Puzzle Fighter 2 HD Remix, Trials HD