Your favorite Xbox live game

What's your favorite xbox live game ever, and for what reason is that your favorite?? Its very hard to pick a favorite, but if I had to, I'd say halo 3. Gotten lots of good playtime from it, all the achievements, and I absolutely love the combat. Very great maps and overall gameplay.

CoD 4 was great on live. All the unlockables and so many different ways to play. Tons of classes etc. combos, you name it. Also, i love versus mode of Left 4 Dead but i bought that for PC to play with friends.

There is NO GAME EVER in which you can own someone as hard as you can in l4d. For instance, anyone ever completely killed a full health survivor right at the beginning, as a smoker or hunter? Or cover all 4 of them in boomer puke? Complete pwnage. They cant do anything except watch themselves get literally shredded. Its a good time.

Also, playing as the survivors and shutting down the Infected is great too. You can totally ruin someones fun if you keep killing them. Stick em in spawn limbo!