I would say Counter Strike: Global Offensive. Because it's one of the few games where teamwork really is key if you want to win. So because of that, I would say that CS:GO is my favourite COOP game of all time.
You Know man. Nothing Can Reach LEFT 4 DEAD 2. The BEST CO OP Game Ever Made. its Too action and coo. I never Forget the "DARK CARNAVAL" campain.
DEDSECali.b Is Watching You 8->
DONT TRY TO HIDE FROM HIM!
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My favorite is Borderlands 2 or Portal 2! Can't pick between the two of them, but I know I've always had an awesome time staying up late during the summer playing those games with my friends ^w^
EtrosKnight <-- Border 2 and Portal 2 are great games!! I finished 100times both of them.. but best COOP is for me Trine 2 that story and effects are awesome