I've played in several since we started played 3E ...
Freeport (plonked into Greyhawk) -- I really like the wide-open feel of Greyhawk (contrasted with Forgotten Realms), and I began my 3E DMing as a big fan of Freeport. As Freeport developed, it started getting a little too silly for me, but before I could pull the plug myself, I had a TPK (in Black Sails) when the PCs were 10th or 11th level. I still love Greyhawk, but too be honest I'm not sure how much of that love is simple nostalgia.
Scarred Lands -- I just dug the riff on Greek mythology that went on in this setting, and I really liked being in from the beginning of a campaign setting with massive ongoing development. This setting has my favorite D&D nation ever: Calastia, the realm of a beloved LE ruler. Although Scarred Lands was 3.0, I recall the setting books being almost entirely fluff, so if you can get them cheap I highly recommend them. I TPKed my group at about 6th level (ironically, in a module written for Forgotten Realms).
Eberron -- It shocked me how much I enjoyed Eberron, because I'm sort of a fantasy purist. For example, I very much dislike psionics and other science-fiction elements in my D&D. (I even had a hard time with the firearms in Freeport.) But I loved Eberron, somehow, and I would say that as of now it's my favorite setting. The history and national identities are very interesting to me, I'm enamored of the slight shading of D&D's normal black vs. white morality, and I enjoy the new rules (like the races). Even the setting's quirks -- e.g., the map scale is off by a factor of 10, according to Keith Baker -- are somehow charming.
Golarion -- I haven't played in this one yet, so I don't feel I can make a wholehearted endorsement. I will say, though, that judging from what I've read -- which is about a quarter of the setting material published -- Golarion might supplant Greyhawk as my favorite "generic" D&D world.
Forgotten Realms -- For quite a while, I was genuinely angry at the love shown for the Forgotten Realms -- in production values (the FRCS ... are you kidding me?) and in production schedule -- but aside from that, FR just doesn't do anything for me. Too developed, too nit-picky, too ripe for GM-NPC garbage.