#3. Borderlands
Dozens of Co-Optimus forum users can't be wrong: Borderlands is one of the top co-op games available today, even a year after its release. Teaming up with three others to roam the apocalyptic world in search of gear and experience is quite addicting. While lacking splitscreen online play, the remarkable support for the game keeps players coming back. Some of the best downloadable content you can get is found in the wild world of the Borderlands. Let's just hope that a Borderlands 2 is coming someday, so another legendary co-op series can be initiated.
#2. Gears of War
If imitation is the sincerest form of flattery, then Gears of War has recieved an embarassment of praise. It entwined third person action and the cover mechanic to absolute perfection, and is one of the most influential games of this console generation. While the first game limited the co-op to two buddies, Gears of War 2 upped the limit to five players in Horde mode, a gameplay mode that's fun to revisit again and again. As if chainsaw machine guns weren't enough to keep you coming back. We look forward to Gears 3 curb stomping our minds once again next fall.
#1. The Halo Series
Can anything top Master Chief? Not much, except maybe a smaller fellow known only as Noble 6. Halo Reach is the pinnacle of cooperative gameplay on the Xbox 360. Four player co-op through both campaign and Firefight modes, including two player split-screen, is simply glorious. A staggeringly robust rank system and weekly challenges extend the experience as far as you want to take it. Don't get us wrong, the other Halo games on Xbox 360 are among the greatest available, too. The culmination of the Halo story (so far) in Halo 3, the successful genre-shifting of Halo Wars, and ODST's Firefight mode (plus my favorite narrative structure to boot), along with Reach, make Halo the cream of the co-op crop for the Xbox 360.