The Fable games spring to mind immediately, Fallout 3, Elderscrolls 4 Oblivion (and Elderscrolls 5 Skyrim later this year) Mass Effect series, Lost Odyssey (made by the FF developers but not on Squenix rolls) Last Remnant, Infinite Undiscovery, Star Ocean (theres only one on 360) and the Dragon Age games all come to mind immediately, theres also the marmite status game of Too Human.

Outside of the above RPGs I will say:

Burnout Paradise
Red Dead Redemption
Dead Rising (1 & 2)
Call of Duty Modern Warfare & Black Ops (if only for zombies and multiplayer)
Halo Wars
Civ Revolutions (hellz yeah!)
Saints Row 2
GTA 4
Gears of War
Ace Combat 06
Assassins Creed


Are all essential to own games.