Although I haven't been in many:
Worst: The 'temporary BA lounge' in Terminal D at DFW. Even the lady controlling entry apologised for it before I had even set foot inside (She was right!). It was dark, dingey and horrible with inedible snacks and no hot food. I hope the new one whenever it opens is a lot better (to be honest I quite liked the old one).
Best: Boston BA lounge - Everything the temporary DFW lounge wasn't.
BM