I've played most of the settings listed, with the exception of Spelljammer and Dragonlance. Greyhawk is and will probably always be my favorite for a number of reasons. First, Greyhawk is all about D&D. It is a fantasy world designed for the game and hence, it has something to offer almost everyone. But one of the best parts about Greyhawk is its adaptability. With Forgotten Realms, you have a lot of players who will be steamed if you retcon the Time of Troubles and say all the gods survived. They read the novels and they know the stories and play the game to participate in stories they already know (maybe not all of them, but that is my general impression of FR players). However, with Greyhawk, there is no huge body of literature or detailed source manuals to tell you what color bricks are used in the buildings of the southside of Safeton. Who has ever even HEARD of Safeton? Exactly.
Greyhawk is a skeleton. It isn't the most heavily detailed campaign setting around, but it is a consistent setting and one that is easily malleable to the DM's wishes. You can run almost any kind of campaign in Greyhawk and make any number of changes without grossly disrupting canon. In that way, every DM can make Greyhawk his own and provide a unique experience for his players. Giving the DM flexibility is what makes a really great D&D game, and Greyhawk is designed for that, so it's consequently the best campaign setting.