Fallout 3 (Platinum Collection)

Bethesda took their immersing world building RPG-FPS talents honed in Oblivion and applied them to a post-Apocalyptic American Wasteland. Fallout 3 is melds great RPG leveling system & expansive open-world design with FPS game play and plopped them into an expansive, open, engaging setting.
Set in a post-apocalypse America, Fallout 3 provides well over 100 hours of main and side missions for the player to complete at his/her discretion, with enemies leveling up along with you.
With the DLC available and at the Platinum Edition price, you really have no excuse not to have Fallout 3 (2008 Game of the Year) in your 360 library.