In order of discovery, and naming only those about which I still feel positive (oh Dragonlance, how dull you seem from my present vantage):
Spelljammer. I like the science fantasy, Age of Sail vibe - tooling around in a wholly unrealistic version of outer space having wild adventures.
Planescape. I'm really attracted to the premise that people on the Outer Planes fight more over ideas and beliefs than they do over land or gold. That, plus the out-there locations that you can visit. My ideal Planescape campaign would play like a more serious, philosophical version of Labyrinth or Mirrormask.
Eberron. Just fantastic. Pulp adventure, noir intrigue, fresh and worthwhile takes on standard D&D tropes like races and classes - plus, as an added bonus to me, plausible and interesting religions. Oh, also airships!
Well, that's D&D settings only, of course.