I voted Greyhawk, Eberron, and Ravenloft. Greyhawk has always been my 'beer & pretzels' setting that I use when not running my homebrew -- nice setting. Eberron has my attension right now, but I'm not sure if it'll hold long. I used to buy all the Ravenloft stuff in 2E and swiped almost all the horror mechanics, but the 3E stuff from S&S hasn't caught my fancy, so there's a caveat to my choices.
Some products for other settings might interest me, but they wouldn't be regular hits. Everyone in my group really likes the concept of Dark Sun, but it never seems to make it into our rotation of games; and the suppliments are unlikely to have cross-over potential. Al-Qadim probably has some stuff worth looting, but I've never really looked at it. Ditto Counsel of Wyrms and Birthright.
Strangely, the top two picks are two of the three (Spelljammer being the third) settings I like least.
You couldn't pay me to buy Realms stuff -- okay, you could, but it wouldn't be cheap, and they'd never actually make it to my shelf. I can't tell you exactly way, but the setting just turns me off. I dislike it enough that I don't even want to loot mechanics from it.
Planescape seems like a fine setting, but most of the things that fans say "cool" about just turn me off. I just don't like plane-hopping. IMO, the Prime is the center of the multiverse and the only reason for any other planes to exist is to support it. Unlike Spelljammer and FR, though, I can see why others like Planescape. I'm just not in the target audience.