This Role-playing shooter game perfects co-op and has a hilariously good story.
The story starts out with Claptrap finding you and taking you to his house. A monster (I think it was a Bullybong but I can't remember) comes in and takes his eye. You then have to go get it while protecting him too. This is just one of the many outrageous and memorable quests in the game. The story focuses on the bad Handsome Jack trying to find and kill you. It had really good writing, much better than the original's.
The multiplayer for the game is excellent. It improves on the already excellent co-op and adds in a lot more guns. It has never been as much fun driving around killing monsters with your buddies.
Borderlands 2 is my favorite shooter of the year. It had a lot to offer and will keep you busy for a long time.