The worst for me are:
#1 Planescape- the "berk" this, and jargon was just too over the top. Plus the planes became mundane, and the setting's outlook was far too modern and unified to represent the vast unknown that should be the outer planes. I know a lot of people love it, and what drew them to it is what drove me away from it. And the artwork was an absolute turn off for me- I liked some of RK Post's stuff for it, but loathed DiTerlizzi.
#2 Dragonlance- the ultimate railroad campaign world, it has basically ONE plotline per "age" where the main movers and shakers are the only characters that matter, and everyone else is an extra. Its a decent vehicle for novels, but absolutely terrible for a campaign world- and I tried running 3 campaigns in it, so I know.
#3 Forgotten Realms- the problem here is information overload. I liked the original grey box version of FR, but past that it bloated like a dead whale, and was not internally consistent (ie, too many developers). You can run a decent campaign in one corner of the world if you ignore the rest, but trying to keep up with canon, the ultra-high magic levels, godly NPCs, and fending off fanboy players when you change something made me toss in the towel on FR long ago.
#4 Eberron- I tried to like it, read the book, and while it has some interesting ideas, its "more D&D than D&D" or as another poster put it, "FR on crack". Including everything and the kitchen sink, while good from a marketing perspective for WotC, made the world seem contrived to me, and the cultural outlooks and beliefs of people in the setting is far too modern for my tastes. It also seems like the ultimate power escalation setting- but time will tell.