20 Which setting have you enjoyed most? Why?
Published or homebrew? And non-D&D (because really, the most fun we had back in the day was with the Known World of the Expert Set/Isle of Dread, and more recently, I've had some good times with various other D&D worlds, including our sorta stalled out Vaults of Ur game).
Let me think of what non-D&D campaign settings I can think of that got more than just a session or two of play (sorry, WEG Star Wars, as much as I love original trilogy SW, I never got to play it much).
The Frontier Sector – Star Frontiers
Marvel (616?) Universe – Marvel Superheroes RPG
Gamma Terra – Gamma World
Teenage Mutant Ninja Turtles – TMNT and Other Strangeness
Prequel Star Wars – d20 Star Wars RPG
Not sure if it has a name, but the dystopic future of Trinity RPG
Arkham, Mass. and environs – Call of Cthulhu
Aliens/Predator-verse – d20 Future
and tons of D&D settings (homebrew and canned) or settings that never saw play beyond one or two sessions (and actually I'm trying hard to remember if the Marvel game actually got that far or not...)
If I had to pick, though, it would be Gamma Terra. From my first introduction to it through the Endless Quest book Light on Quest's Mountain to our early 90's game with the old BECMI group, to my more recent games with the Busan Board Game Group, it's always consistently provided a strange, funny and dangerous place to adventure.
The Frontier Sector would have to come in second just from sheer numbers of sessions played within it.