Ten? Eesh. That's really tricky.

Spira, FFX
I don't like FFX all that much for a few reasons but Spira is freaking gorgeous. It's exotic, magical, mysterious, beautiful... such an incredible world to explore.

Gaia, FFVII
From just a world map point of view, I adore this world because it inspired me to make my own maps, something which I love doing. It had a varying technology, cultures and lifestyles. The materia/lifestream system fascinates me to this day. There are lots of little places in this world which I really enjoyed seeing and exploring.

World, FFVIII
A world with no name! This makes the cut for variety in towns/cities and the Shumi/Moomba species. Freaking love them, man. I also love the individual locations a lot, such as Deling City or Balamb.

Arcadia, Dreamfall/The Longest Journey series
A truly magical world, but with real world politics and all the beauties and horrors that combination allows.

Gran Pulse, FFXIII series
Utterly gorgeous. I really wish the new games heavily expanded on the background of the world because I would have loved to go back to the "old times" and see what life was like on this planet. Or explore the planet in new times to learn more of the past. The beauty of the world combined with the tragic fate it went through, and the huge potential for story that the mythology allows for... missed opportunity much!? Still, love it.

Hydaelyn, FFXIV
This world is utterly gorgeous and has a bit of everything. It has maps vast enough to really bring out the best in itself, and reward those who explore. And then you reach the more fantasy-like environments and yeah, it's just my kinda place. What a real fantasy world should be like.

Ivalice, FFT series/FFXII
Largely meriting it's place because of what we see and experience in FFXII. How come we don't see more games with seasonal landscapes and stuff like that!? I had so much fun wandering around this world. And the tribes, particularly the Bangaa, have always captured my imagination.

Nirn, Elder Scrolls series
God damn this world is massive and beautiful. Morrowind is the a treasure hoard of fantasy landscapes. Oblivion gets a bit samey but it's just one region. Skyrim really blooms. When I heard it was mountains and stuff I was skeptical because snow landscapes are very much not my bag. But then I explored the world and oh man, I really need to explore it more sometime, it's just got so much to offer.

Myst, Myst
I don't care, I'm going to pretend that this is an RPG. x( I mean it sort of is, right?

The last one? Eesh, really struggling at this point to pick which ones I like best. The likes of Vana'diel (FFIX), Gaia (FFIX), Cocoon (FFXIII), the worlds from Grandia and Fable... hard to figure out. I'm probably overlooking some other excellent ones, too.

EDIT: Oh, and I deliberately did not include MiddleEarth because I don't feel like it's fair to include worlds that I know primarily through books.