I voted Mothership Zeta expansion. Hated it. I can't think of a single thing I like about. Well, actually, I like some of the dialog: the little girl has some funny lines. I was never able to finish this expansion. What the hell was Bethesda thinking when they made this one? I've disabled the esm so that no other characters will have to go through it.
Operation Anchorage bothered me a little bit because it was too short and almost too easy (and for me, nothing in Fallout 3 is easy). I've finished it in an evening.
The snow was pretty, though.
And it being fun makes up for it being so linear, somewhat.
I don't understand the hate for Broken Steel: it fixes a very bad end-game bug (OK, not a bug because it was deliberate, but it's stupid, so I call it a bug).
At first, I didn't like Point Lookout, because it's implausible that a mutated descendent of a stereotype redneck should be more powerful than a deathclaw. that's stupid. Fortunately, I found a plugin to fix that. The nearly monochrome look of the place also bugged me, but another plugin fixed that.
This expansion is nice because it's not so linear: you can just wander around, like in the original game.
The Pitt bothered me becuase of the moral ambiguity of the main quest line. No way to do it "right". I like the visual look of the place, though.
p.s. A datum for Bethesda, for future games: the "here you are, without any of your original equipment, and you can't leave until the quest line is done" thing has gotten old. No more, OK?