From all the fantasy books I've read, or those I know things about, the top five are, in order:
1) Arda (Lord of the Rings);
2) Westeros (A song of ice and fire);
3) Hyboria (Conan);
4) Eberron (D&D);
5) Narnia;
I also find pretty interesting the Dark Tower series world, even though it's kind of confusing sometimes - and steampunk, or something near by.