I've always been a fan of invented worlds - Here's my top ten of those created by science-fiction and fantasy authors:
Middle-Earth, created by JRR Tolkein in the well-known trilogy which I don't need to name since anyone that doesn't know it won't be reading this! There's little I can add that hasn't already been said by many others.
Yrth, created by Gene Wolfe in the tetrology "The Book of the New Sun", a richly-detailed and evocatively-described decayed far-future version of Earth, so far in the future that our own history is just dimly-remembered myths and legends.
The Culture, created by Iain M Banks over the course of several novels. A vision of a really advanced civilisation, described as 8000 years ahead of own, a utopia where 'no-one can dominate others through control of limited resources', filled with 40-mile long spacecraft and artificial 'orbitals' giving almost unlimited amounts of living space.
Glorantha, created by Greg Stafford and others. I felt I had to include one gaming universe, this is that. Forget the formulaic cliched faux-medieval DnD worlds churned out by various d20 publishers and hack novelists, Glorantha, with it's deep mythological background and richly detailed cultures is simply the best gaming fantasy world ever. Originally published as the world to go with the RuneQuest system, it's now being republished with the new Hero Wars game, more suited for epic narratives.
Discworld, created by Terry Pratchett. He's written more than twenty books in the Discworld comic fantasy series now, which are far better than the twenty-plusth book in a series has any right to be. The Disc itself has evolved from a simple parody of fantasy cliches into a richly detailed world which is more and more a reflection of our own world seen through a fantasy lens.