My vote goes to Mystara, or as I still think of it "The Known World." I started playing D&D in that setting back in '86, using the red box Basic Set series with the Elmore cover art. I definitely have the fondest memories from that era of play. The old Gazetteer series, before the whole Hollow World and Wrath of the Immortals stuff, still had a flavour that most D&D settings don't have. Even if some of it was a tad silly (GAZ4 Ierendi, for example).
We eventually adopted AD&D (1e, obviously) rules a year or two later, but stayed in that game world. Some of my favorite modules were set in that world. The "classic" 1e AD&D modules (GDQ series, Slavelords, S series, etc) never really did it for me. When I think back to the "classics," I think of B7 Rahasia, B10 Night's Dark Terror, X1 Isle of Dread, X4-5 Desert Nomads, X10 Red Arrow Black Shield (I love that adventure), CM1 Test of the Warlords...
Greyhawk never really captured my attention. It always seems a little bland to me. I have fond memories of the old "grey box" Forgotten Realms set, before the setting was over-developed, over-populated with powerful NPCs, and before hordes of trashy fantasy novels were written and incorporated into the game setting.
Planescape was really cool, but I never really played in it. And I love the setting for Birthright (from what I've seen of it), but haven't ever played in it.
Somehow, for me, the quintessential D&D experience is playing in the Known World with the old Red- and Blue- Box Basic and Expert Sets. And when I think of the look of D&D, I think of the Elmore art from the old B and X series.
If they're going to do a 3.5e "basic set," I hope it harkens back to the old 1983 Basic Set that brought me on board.