In a year with a lot of great DLC, Undead Nightmare stands above the rest. The concept of bringing the increasingly ubiquitous undead into the serious (but satirical) Red Dead Redemption was a bit controversial, but wound up bringing new gameplay to an already fantastic game. Undead Nightmare is, without a doubt, the best DLC this year had to offer.
For more information on the winner, read our review of Red Dead Redemption: Undead Nightmare.
Left 4 Dead 2: The Sacrifice
When Left 4 Dead was first released, one of the few criticisms of the game was that there was no real story to it. However, with the release of Left 4 Dead 2 the universe of Left 4 Dead has been expanded, and The Sacrifice tells the story of the original four survivors, how they met the survivors from the sequel, and Bill's ultimate fate.