My big love is Dragonlance. There's too many individual reasons why, but it boils down to a great high fantasy flavor and storytelling based on the tropes of D&D. Check my sig for the Dragonlance Nexus website.
My first campaign was in the Forgotten Realms, so I like it as well. Also liking Dark Sun, Eberron, Spelljammer, L5R, and Star Wars. I want to learn more about Golarion, Iron Kingdoms, and Al-Qadim. They all look neat.
I wish the Legend of the Seeker TV show was an RPG. That would be fun. Starting to read the novels by Terry Goodkind right now.
I'm certain I'm forgetting something.
I never did get into Planescape. Everybody comments on the art, but I wasn't a huge fan of it. I will say it's distinctive, though. I didn't care for the slang at the time. Maybe, though, this is something I could get into after the fact. *shrugs*
I like elements of Greyhawk, but I think I'd rather pillage them for another setting. Ditto with Mystara. Ptolus sounds like something good to put into a setting somewhere.