Forgotten Realms!
I practically only know GH and FR. And the group I play in is basicly just hack-n-slash, so magic is more than welcome. Therefor my vote went to FR.
Stegger
I voted for Scarred Lands. Strangely the thing that most attracted me to this setting at first was the fact that there were only 9 gods for the PC's to choose from (well 8 gods and a titan).
I have never liked the huge number of gods that seem to populate other D&D settings. They just seem to crossover so much that many are redundant. Now with the Divine and Defeated book out they have added many Demi-gods for the non-human races (although nothing would prevent a non-human from worshipping one of the original 9).
I have mixed feelings about this change. I realize that it was never the desingers intention to leave the world with only 9 gods but I find many of the demigods to 'crossover' with other demigods or gods fields of interest. I'll have to decide which ones are truly represented in my campaign.
But, hey, that's the fun of D&D in general - everyone has their own priorities and likes.
The After Winter Dark Setting by Troll Lord Games.
I like the depth of history and scope. Also the Dwarves were the first comers not those nasty elves , and for a millenia halflings were hunted for sport (they're a little bitter now for some reason).
Originally I had planned to support Greyhawk but WoTC's treatment of it left MUCH to be desired (no offense to EriK Mona and LG).
The best setting for d20, particularly fantasy, is Earth. Yes, Earth. It's the most well-designed and fleshed out world I've seen, and though the rules supplements are kind of lacking, I think there's a really strong fan following waiting to gobble up the books whenever they might be released. Who here is a fan of Earth?
Aquerra is my favorite since it is my baby. I cannot objectively say it is the best - but it is the best for me or else I would have abandoned this soul-sucking project long ago.