Gears of War 2
Let's get this out of the way: I am a co-op shooter junkie. In particular I love any co-op shooter that my brother and I can easily play together, even though we live 1500 miles apart now; Gears of War 2 was a game I picked up in the winter of 2008 after picking up my second 360 (the one my brother and I had bought together had been left with him in Louisiana). I bought it because we had played Gears together and loved it, and hoped that we would like this one too.
I want it understood that I don't play Horde or Deathmatch on Gears. THis game made this list solely for the co-op experience.
I actually reviewed the original Gears of War for gamrReview; you can see everything I loved about the games there. It's loud, it's bombastic, it's juvenile, it's irreverent, and it does everything it can to punch you in the face with exciting imagery and engaging battles. Coordinating with my brother over a headset made for one of the most engrossing and engaging gaming experiences I've ever had, and I would not trade it for anything.
This game's campaign does have one flaw: the fish boss is bullshit. If it hadn't been for that thing, this game would have easily landed in my top 20.