My latest obsession on the Xbox360 is Fallout 3.
Set in the post-apocalyptic surroundings of Washington DC, you play a character who has only recently escaped the underground vault or fallout shelter that you grew up in to brave the perils of the wastelands for the first time.
An RPG with a compelling story and lots of tongue-in-cheek humor, Fallout 3 is a blast of a game. The gameworld is huge and I reckon a full playthrough without exploring everything would take 80 - 100 hours. In other words, know what you're getting yourself into before you start on this game.
The combat in Fallout 3 is a kind of real-time turn-based system called VATS that allows you to target parts of the opponent's body and uses 'action points' as a means of controlling the turns. It's a bit hard to explain here, but it's a lot of fun and it doesn't become tedious (compared to the combat in Fable 2, for example).
I, personally, was a big fan of Fallout 2 and surprisingly Fallout 3 is very similar to Fallout 2 in many ways. If you played the previous game and liked it, it's a sure bet you'll like the new version.
I give this game a 9.7 out of 10, I think it's awesome.