For me the list would go something like:

1. Discworld
2. Dresdenverse
3. Amber
4. Middle Earth
5. Pyrdain

Lots of good series mentioned in this thread. I like The Land from Thomas Covenant but the story arcs turn me off somewhat due to the content and self loathing. I also like Temerant from The Kingkiller Chronicles. And A Wizard in Rhyme by Christopher Stasheff. Of course Xanth by Piers Anthony. Several of Mercedes Lackey's lands are nice. For darker stuff, Simon Green does nice worlds.

- E