Some nice single-player games are Assassin's Creed II and Batman: Arkham Asylum.
MW2 might be nice, but it's short and is half multiplayer.
Borderlands is a pretty good FPS-RPG-shooter.
Fallout 3: GOTY is also a pretty good RPG (First-person and 3rd-person) packed with content.
I heard Dragon Ages: Origin is decent.