Hiya!
Greyhawk for me. Between it and Planescape, Greyhawk is
infinitly more "make it your own". GH, I can run a campaign based on commoners rising up to fight off a hoard of orcs. Or I can run a political intrege campaign around Iuz, the Shield Lands and Furyondy. I can keep it wide-open and just have the goings on of the world...just go on...so the PC's can dungeon-hop all over the place. The problem we had with PS was that it was pretty much hard-coded what you could and couldn't do. For example, in GH I could say "The Circle of Eight disappeared right before the Greyhawk Wars, never to be seen again"...but I can't say "There are no Factions anymore" for PS. The Factions are hard-coded into the setting. Same with the whole Blood War thing (which we all hated...). In GH, new PC's can go explore the world....in PS new PC's can hang out in small little 'safe areas' for them...no wandering the planes of Hell at 2nd level.
Anyway, i was actually just looking at my Planescape stuff last night.

Now that I've had just over 20 years to get used to it's ideas, I can see why so many folk like it. I was thinking of doing a small PS mini-campaign...to see if I and my group could "get into it". But I can only see it as a mini-campaign because it is so dang...
specific and, as I said, "hard coded" into.
Love Greyhawk...know it pretty dang well, having DM'ed in it for 36 years now.

I can appreciate Planescape
much more now than I did 20+ years ago when it came out. I think I'm ready to give PS another shot now, but for "best D&D campaign world"? No contest. Greyhawk. Period.
^_^
Paul L. Ming