Spelljammer, followed by Eberron and Dark Sun. I'm on the fence about Mystara, because while I'd love to see the setting have official support, I wouldn't want to cut the fan community-- including Bruce Heard-- off at the knees.
(Almost) ALL of them.
In no particular order (except alphabetical, it turns out):
Birthright
Dark Sun
Dragonlance
Eberron
Planescape
Ravenloft
Spelljammer
Each has aspects I want to steal for my own homebrew setting.
Dragon Kings is not Dark Sun, though it is by the same creator and inspired by same. WotC own the Dark Sun setting (not its creator) in its entirety, so producing a 5e guide for it is entirely possible and likely.
Jargogle
I would like a rebirth of the "Nerath" universe, in a single volume this time.