I've been visiting Osted Ard (Tad Williams's Memory, Sorry, and Thorn and the more recent follow on series Last King of Osten Ard) since I was 11, so that will always have a special place in my heart. As too will Krynn, the world of Dragonlance, because of the (literal, I'm not exaggerating) decades I've spent there playing AD&D.

But, if I had to pick just one fantasy world or universe: Michael Moorcock's Multiverse. No question. The themes, the richness, the variation, everything about it.