I could have swore that I posted mine a while ago, but I don't see it.
Gears of War. If you only need 1 good reason to get an XBox Live Gold subscription, this game is it. There are always tons of games going at all hours of the day (trust me, I know). Hell, I still haven't gotten sick of the Campaign of this, but the multi-player world is really where this game shines.
Gears 2 is going to be 10x better in every way though. I seriously cannot wait!