Bethesda has had the two worst pieces of DLC:
1) Horse Armor
2) The ending of Fallout 3
Yeah, whatever, I can see people saying it ended fine, but the first piece of DLC was a continuation of the main storyline and wrapped significant up loose ends. DLC should either not be part of the main story line or be a tangent of it.
I'd also say the Operation Alaska, or whatever, holgram flashback Fallout 3 DLC was completely terrible as well. It ditched the role playing and went right for combat, which just made it not very entertaining.