hmmm, there have been some really good DLC's from Bioware and Bethesda...
I would have to say My favorite DLC is still lair of the shadow broker from mass effect 2, with the citadel dlc from mass effect 3 being a close second.
I loved every piece of dlc for fallout 3 as well, and skyrim dlc was also good, but slightly overpriced for what it was...