Either Skyrim, Portal 2, or Arkham City. I myself go with Arkham City, but I haven't played Skyrim at all.
Portal 2 was the best game of the year just from its single player campaign. Adding on the excellent co-op campaign was just icing on an already delicious(lying) cake. Ask to speak to life's manager! **** lemons! Honorable mentions for me : Shadows of the Damned, Dead Space 2, Bulletstorm. Skyrim probably too but I've only played it 5 hours.
Saints Row 3 As much as I loved Skyrim(insert overused arrow/knee joke) and Portal 2(SPAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACE) thoes games didnt let me play as a fat zombie cowboy fighting luchadores with a hover bike and a oddly shaped bat
I kind of agree with you there I found that taking away whats little srs 2 had made all the silly stuff seem off putting (and two of the games deaths felt really really rushed) I still had more fun running around in steelport than I did luring guards to get hit by giants in skyrim but thats just my opinion to each his own and all that
they added alot of new cool features and modes Community made modes like left 4 dead did with mutations Survival