This thread is by no means limited to D&D, but my comments are based upon D&D campaigns that I have run or played in.

For me, I'd have to say it was Greyhawk.

It lent itself more to the low-magic campaigns that I favored, and I also liked the predominantly european medieval flavor of the setting.

While it wasn't fleshed out in nearly the detail that the Forgotten Reams was, Greyhawk seemed to be more DM-friendly in terms of creating adventures as well.

Also, mythology takes a more reserved role in this setting, which I thought gave DM's more flexibility.

The Werefrog love Barsaive. It's a wonderful little setting that explains in an almost believable manner why there are dungeons dotting the land, why said dungeons are filled with monsters and treasure, and why there are adventurers going to claim said treasure and killing said monsters.

I like the twist that Dark Sun applies to fantasy tropes, the way belief is explored in Planescape, and the expansiveness of The Wheel of Time.

I like Planescape, but I want to point out how cool (the first edition of) Mage: the Ascension was. Planescape talked a lot about how belief influences reality. M:tA actually fleshed that out in terms of game mechanics.

Oooh I could not disagree more. M:tA drove me crazy. Planescape is a work of beauty though.

Paradox, are you kidding me? Oh the rage filled debates we had

I like Mage, from what I understand about it. I never got a chance to play and never owned a book, so I'm probably waaaay off.

Edit: Oh, and Planescape of course.

I really like the level of detail in Forgotten Realms, even though it's a nightmare to run. FR setting material makes for great bathroom reading.

For a game where you all play Mages, my recommendation would be Ars Magica. Or Unknown Armies.

I do love Mythic Europe. A little time on wikipedia can give you a ton of really cool ideas based on real legends the players have probably never heard.

I've never run across one I like enough to want to run a game in. I always make my own. My current one is my favorite, though it's not very standard.

From a purely setting standpoint Exalted was pretty neat.