I love Dark Sun. I even love the backstory, though the events of the Prism Pentad need to be forgotten as quietly as possible. The low-level survivalist brutality and high-level epic savage grandeur are something to behold. Lynn Abbey's Rise and Fall of a Dragon King is the best illustration of 'my' Dark Sun.
I love a very particular version of Ravenloft - the 3e version that the Kargatane were exploring through the Gazetteer line for S&S, before all that went pearshaped. When Ravenloft was starting to actually become a world of its own, rather than just a loosely-connected series of settings for one-shot adventures. Probably the best example of D&D at a slightly advanced technological level, imho, though obviously hampered in implementation by the limitations of the d20 system.
I love bits of the Forgotten Realms (the 3e version), and the further from the ever so samey Waterdeep-Dalelands-Cormyr heartlands they are the more likely I am to love them. Mulhorand, Unther, Chessenta, the Vilhon Reach, the Great Rift, Anauroch, the Shaar, the Golden Lands, the Shining South, Rashemen and Thay. but FR is so varied that if you can't find SOMETHING you love in there then I'm inclined to believe you're being curmudgeonly for the sake of it!
I love Zakhara. Genies and swashbuckling and good manners and lovecraftian horror. Such a perfectly designed world for the sort of games that Al-Qadim was intended for.
I love the Iron Kingdoms, though as an RPG setting they've gone a bit the way of the Warhammer world in recent years - the necessity of strongly branding and differentiating the opposing factions for a tabletop wargame makes everything very all-or-nothing and cuts down on the roleplaying possibilities provided by ambiguity and cross-cultural interaction.
But having said that, i love the WH40K universe in all of its gonzo grimdark illogical hopelessly tyrannical glory.
I love Castle Falkenstein.
I love Vampire: the Masquerade, just by itself, without any of the other zillion world of darkness critters that eventually made the setting collapse in on itself under the weight of its own inherent contradictions.
I love most of Planescape, though if I were ever running it I'd probably find a reason to severely de-emphasise the factions. But the sheer scope and breadth and possibility of a setting where the PCs can encounter anything or go anywhere that might conceivably exist in the multiverse, no matter how improbably, is very appealing.
I actually liked Maztica, even though it was a massive historical ripoff, but unfortunately they blew it up in the first novel trilogy - before the first boxed set was even released! What a waste. It never recovered after that.
I quite like Golarion, but it's very much what it was designed to be - a setting where there's a nation that allows you can play any of the 'standard' D&D games - gothic horror, jungle exploration, Arabian, Egyptian, viking, etc, etc, etc. It's very solid and some of the detail is magnificently crafted, but it just seems to be lacking that little bit of mad inspiration.