Favourite Published?
Dark Sun would be my setting of choice. I'm always a sucker for post-apocalyptic settings, as they have adventure built right in - and there are plenty of logical places to put "dungeons" without them seeming fake or ill-fitting. What's funny is that I've never been super attached to the idea of Dark Sun in a desert - I really believe it wouldn't be too hard to do Dark Sun on an ocean. Or Dark Sun in the arctic. Just so long as the accumulation of power is related to the idea of making the ecological situation worse, the setting works.
Jakandor is my #2 setting of choice. Everyone forgets about it, as it came out in the final days of TSR and wasn't really advertised that much. But it was pretty frakkin' cool, and 4e is the PERFECT engine for it. Humans-only world. The Charonti are a highly-lawful, undead-using mesoamerican-style group of wizards trying to recover their ancient culture, including the rediscovery of long-lost spells. Meanwhile, the Knorr are a native american/viking cross of highly shamanistic hunters and barbarians who are religiously anti-magic, and believe that "civilization" and law weaken the natural spiritual strength of man. Naturally, both sides hate the other. And neither side is evil or good. Brilliant setting... and also very post-apocalyptic.
Council of Wyrms is pretty fun, because you play dragons. And being a dragon is pretty awesome... and then you realize that everyone else is a dragon, too. While you get to have fun bullying around pesky mortals, there's also a lot of politicking because you don't want to take on the other dragons. Plus, since age is a big part of the campaign, there could be a period of a century between adventures - which allows the GM to showcase the forces of history changing the world around the PCs.
Earthdawn, while not a D&D setting, is another one I'm pretty keen on these days. Highly-magical, and also sort of post-apocalyptic, the namegiver races have weathered five hundred years of a magical catastrophe, holed up in scattered underground dungeon-cities. Now, on the tail end of the catastrophe, they are emerging into a changed world and rebuilding. Neatly done, and the first setting I had ever seen where humans were not the dominant race, either numerically or politically (I actually believed this was a bad design decision when I first saw it, as the 2e DMG told me so. Now I realize just how stupid I was).