I have played 3 and owned 2 of them and heard about the other.
I've owned Gears of War 2 and Halo 3 Played Gears of War, Halo 3, and Resident Evil 5 Heard about Left 4 Dead
All of my friends who have owned Left 4 Dead have sold it because they said its a short game and nobody plays it.
Gears of War 2 I sold back to GameStop because it wasn't for me, but its a good game if you have friends to play it with.
Resident Evil 5, i've played the demo and its just not the game for me, but a lot of other people like it
Halo 3, that is the one game out of the 4 that I own and haven't sold yet. Halo 3 I don't think I will ever sell and I think it is the best out of the three.
If you need any suggestions for games I suggest you get Call of Duty: World at War, that and Halo 3 are the best games for the Xbox 360 ever!