After shooting our way across the southwest, through Mexico, and back again in the increasingly desperate main quest, "Undead Nightmare" offered a nice way to return to our gunslinger ways without any pesky morality issues. That applies to both the living and the dead. The honor meter is gone for this new single-player adventure and all that really matters is saving your family from zombification; it's a classic. Help the survivors and hope they return the favor, or knock them off and take what you need. Though the storyline proper lasts only a few hours, this add-on does an even better of job of turning you loose in the Wild West to raise hell. Oh, and you get to try and break the four horses of the Apocalypse if you want to for your steed.
Runner-up: Mass Effect 2's "Lair of the Shadowbroker"
Arguably the best quest for our best RPG of 2010, Mass Effect 2's "Lair of the Shadowbroker" pairs Commander Shepard with one the most intriguing characters--and for many players, love interests--from the original game, Liara T'Soni. Their conversations during the course of this mission prove to be the dramatic high point for the series to date.
While it probably doesn't compare, I was really impressed with BFBC2 Vietnam...and I was not a supporter of the original game by any means (since the game ran like Crysis and the guns had unpredictable recoil)