30 Days of D&D Day 4: Favorite Gameworld

Favorite Gameworld is a tough one for me because a lot of play at my table, regardless of DM, has been what's referred to as "homebrew" worlds. Meaning, the games don't necessarily take place in a published Dungeons and Dragons setting. So, I'm going to have to have a couple of answers for this one, actually.

Favorite published gameworld is a split between two I've never really played in (a single game session or two each, not games that took off): Ravenloft, and Planescape.