Well, I've always had a soft spot for the Realms, and has definitely been my favorite setting since I started D&D nearly 10 years ago.
Dark Sun is my number 2 setting, and I wish they get a real 3.5 update for it (3rd party or otherwise). Athas.org is great, and the stuff in Dragon and Dungeon was useful, but I want a DSCS hardback .
Rounding out my favorites are Planescape, Ravenloft, and Dragonlance. Greyhawk is a good one, too.
Haven't really tried out Eberron yet. I got the campaign setting and the Shadows of the Last War adventure. Just haven't had the chance to try a campaign there yet. From what I've read out the the ECS, I think I'm really gonna like it .
If we want to go non-WotC settings. The Scarred Lands is definitely my favorite non-WotC.
Neither's Conan, Dawnforge or arguably even Kalamar, all of which have been mentioned.
Admittedly not in the poll itself.
Anyway, with that said, I do believe a vote for a homebrew isn't worth mentioning; obviously, most people with a homebrew will prefer it to a published setting.
The poll could do with some more options, though: An "Other (but not homebrew)" for those published campaign settings that fall through the cracks, as well as a "None" for those who like their homebrew and nothing else.
With that said, my vote? It's a toss up right now. Planescape and Eberron are both nifty, whereas Midnight is as well. I'd peg most fantasy d20 settings for 3rd edition as "D&D" settings. Official, though, that's another horse altogether.