[zakhrin]

Mine would be:

Middle Earth
The Death Gate world (Weiss and Hickman)
Lyonesse (Vance)
Amber (Zelazny)
Lankhmar (Leiber)
Osten Ard (Williams)

[juzzza]

Bas Lag (New Crobuzon and Armada) China Mieville. Talk about rich worlds and incredible species and races.

Overworld is great (MW Stover), I especially like the idea of elven whorehouses... Did I say that outloud?

Of course Middle Earth.

I like the world Gemmell creates for his Drenai Series and his Shannow books.

[Blackwing]

Amber, Narnia and Jacqueline Carey's world Terre d'Ange.

[JamesL]

Mine would be Allansia and the Old World from the Fighting Fantasy series (some great imagination at work there), Faerun from AD&D (the world used in the Baldur's Gate games - the atmosphere and history/lore was so absorbing), Tamriel from the Elder Scrolls Series (again, atmosphere was great)...

Mmm. Just realized these worlds are all from video games or old-school pen-and-paper gamebooks. Let's see...from the world of literature, I would have to say Martin's world is great, I quite like The Four Lands from the Shannara series and Midkemia is pretty cool too. Oh, and Middle Earth f course. Can't forget that one. ;)