Gears of War 3
Gears of War 2 really popularised the co-operative wave-based survival mode with Horde, so it's no surprise to see it back in Gears of War 3 and as brilliant as ever. New features include Beast mode, which allows you to finally play as the Locust, and (finally) a four-player co-op campaign. There's also a new 'Arcade' mode which includes user-definable mutators and score tracking, so you can bring a little bit of competition to your campaign cooperation. As far as third-person shooters go, they really don't get much better than Gears of War 3.