Assassin's Creed. Just the first one if you never played it. The game has a few bad design issues but from what I've seen on the 360, the game is very good overall with it probably being one of the harder Assassin's Creed games.
Final Fantasy XIII, pushing on the price, but it is good if you don't mind it being linear.
Fallout 3 is good, just be careful of buying the GOTY edition over regular when talking about the used copies. I would also recommend New Vegas, but get the GOTY edition for that one instead.
Dragon Age: Origin + Awakening, packaged together now and will definitely take quite a bit of your time to play.
That's about all I know before I need to dig into racing games.
Final Fantasy XIII is pretty cool, but it being so linear can make it pretty annoying. I'm looking for something to play to balance that out. Fallout seems pretty cool I remember hearing about it a while back before I had the 360, I think I'll check that out. Thanks.
Yeah, if the linear gameplay will bug you, just avoid Final Fantasy XIII all together. Fallout 3 is pretty much the opposite of Final Fantasy XIII in that it is not linear (except for the tutorial level, which isn't that long) and is part shooter and part RPG. The rep the game got is about right.