Borderlands 2
It’s like Diablo, except it’s a shooter. With robots. And in space. Okay, so not really all that much like Diablo, except for the obsession-forming loot system.
Borderlands 2 is all about the guns, really. And there are lots of them. Hundreds and hundreds of them. You can find guns that shoot other guns out of them, for god’s sake.
The greatest part of Borderlands 2, though, is co-op play. Running around Pandora with a couple friends wreaking havoc is one of the most enjoyable (and unintentionally hilarious, at times) experiences in multiplayer gaming in recent memory. You do have to put up with Claptrap, who is basically R2-D2 possessed by a Gilbert Gottfried demon, though.