Xbox360 - Staple games

What’re your absolute must-plays or staples for the 360, or games that are OG games playable on 360?

I wanna pick up some games because I still have my old Phat 360 that still works and I wanna play through some games I never played from years ago, and games are piss cheap nowadays. I went to the local used game/movie shop and picked up 6 games for like $38 with tax; and all the discs were nearly IMMACULATE (they must have a disc refurbisher at the store?) with no scratches.

Besides Halo2, Halo3, Reach, CoD4, and MW2, what’re you guys playing these days? Those are basically all I ever played. Looking for RPGs, FPS solos, multiplayer; anything really.