Forget Halo 3...If I ever get an Xbox360 it's because of this game Mass Effect (a role playing game) from Bioware.
It's not the action that's appealing (although it looks like they put a lot of effort towards that); it's the story, voice acting, the entire experience that draws you in like a good book. The Bioware guys are the best in the business for this. (I have their two Xbox games: KOTOR and Jade Empire)