As far as D&D settings go, I'd say Eberron and Athas (Dark Sun). I do like Paizo's Golarion (Pathfinder) as well. I haven't read a lot of fantasy literature but Middle Earth wins my vote there. Narnia is a close, fantastic, second.

On the science-fiction front, if we can go there, I'd say my favorite are, in order: 1) Herbert's Dune universe, 2) Marc Miller's Traveller (3i or earlier periods), 3) Niven's Known Universe, 4) Asimov's Foundation, 5) Lucas' Star Wars universe, 6) GW's 40k universe.

So. Yeah. I guess I'm more about SF than fantasy.
M