Goodman Games

Currently it's Eberron.

One campaign setting I've not seen mentioned here is Fantasy Flight Game's Dragonstar: http://www.fantasyflightgames.com/dragonstar.html

When our 3.0 characters reached epic level, we jumped over to the Dragonstar universe and I enjoyed it very very much! It's the perfect blend of sci-fi and fantasy - for instance: Starship engines powered by arcane teleportation spells.

Unfortunately, most of my players adamantly refuse to even consider any sort of sci-fi campaign. Curse them!

Otherwise, I'd have to go with the World of Aereth as my current fave.

It's a hard call. My favorite campaign settings to run are always my own. On those rare occasions when I get to play instead of DM, I really have enjoyed Planescape and, of course, the Realms. We played in Azeroth (Warcraft) before WoW ever came out, and I enjoyed that one, too.

We actually tried Synnibar once. Believe it or not, I enjoyed the concept behind the setting. It was unique. If the game design hadn't been so horrible, and much of the flavor text so... pick your favorite derogatory term... I think it might have done better.

Greyhawk. Now and forever.