Either Crysis 2 or Battlefield 3 I guess. Crysis 2 the clan I used to play with pretty much was the most dominant experience in gaming I've ever had. We were all so dialed in to that game, we'd literally get mad about dying once or twice throughout a whole match. I'm fairly certain that we were partly responsible for death of that game on 360, cus we no lifed it so hard, and I wouldn't hold it against anybody who played against us to just say fuck it and quit.
Battlefield 3 we weren't on that kind of level, but we did win a UK based tournament on 360 in our second try, and I was team captain for that, which was pretty cool.