In my opinion, Gears of war is the best game in that list. A truly phenomenal game, with one of the best coop experiences around. Only Gears of War 2 looks like it could better it.
With Call of Duty on Xbox 360, COD2 is my favourite. An immersive World War II experience. COD4 would be next, a stunning game, but the single player is not quite as good as COD2, however the multiplayer more than makes up for it. COD3 is the worst, rubbish online, and a fumbling, slightly dull single player.
Regarding Halo, the original is still the best, and a classic piece of gaming history. With all the talk about Halo 2 making online gaming popular, it still doesn't detract from the fact that Bungie dropped the ball with this game, and made something that is only just above average. Fortunately Halo 3 is back up to par, and offers a fantastic campaign and coop play, but not quite as stunning as the first one had been.