My selection is Faerun. As both a player and sometimes DM, I love the richness and depth of the Forgotten Realms. Nothing against Greyhawk, as it is always fun to see the famous names and locations when poring over the maps. And Rich Baker's Birthright campaign is cool in a quirky sort of way, although I mainly used it without delving into the whole Domain and Regency aspects.
If I can generate a campaign from scratch before I die, that would be awesome. The follow-through is the stumbling block for me, but stranger things have happened.
I would be terribly remiss if I did not mention Beloterra, my favorite homebrew world created by Ned, our longtime DM. The hand-drawn maps are wonderful, and it's fun just to break them out and look them over once in a blue moon. And when you consider it was populated by AD&D modules, Judges Guild adventures, some Harn, and of course Ned's own creations, it was a world never short on things to do.