This thread seems to have attracted all the votes it’s likely to, so here’s the final tally:

homebrew - 11
Eberron - 9
Spelljammer - 6
Planescape - 5
Mystara - 4
Forgotten Realms - 4
Nentir Vale - 3
Dragonlance - 2
Greyhawk - 2
Dark Sun - 1
Golarion - 1
Shanarra - 1
Birthright - 1