Dinotopia. When I was young, the first book grabbed me and didn't let go. The art was incredible, truly amazing stuff compared to the majority of children's books at the time. I was fascinated by the big-scene environments even more than the amazing dinosaurs. And a world where everyone lived side by side, carnivores and herbivores alike, using technology in harmony with their surroundings rather than exploiting the environment for short-term benefit - it really was a utiopian vision.
Avatar. As messed up as the world of Avatar is, I'm willing to live there just for the chance of bending.
I'd be happy as a non-bender, mind. Their world's not perfect, but it's much less sexist, racist, and environmentally damaged than our own. What I wouldn't give to have a hut on Whale Tail Island, or to ride in a sandbender sled across the desert, or to lie on some black sand beach at the end of the Fire Nation atoll, watching the elephant koi sparkle in the sun. At least you can drink the water in most places without worrying about heavy metal contamination.
other tabletop (please comment)(RMDC, I'm lookin at you) Golly, what an invitation!
Those who read my blog on occasion know that my partner and I have invested a lot into our D&D derivative setting, the Folly. Pretty much everything I said about the Avatarverse applies to the Folly as well, including the black sand beaches. But for non-D&D, I'd have to go with the funky Sailor Moon / King Arthur crossover RP my partner and I ran a few years ago. It technically takes place on Terra, of course, but it was a decidedly cooler version of 5th-century Cornwall and Wales than the real one, to be sure. Wiseman as a villainous Merlyn, Beryl as a sympathetic Mab / Morgan, and no Arther - him having been replaced by the PC, who bore Saturn's mantle. All the Senshi and the * made appearances as knights, notably Rei as Sir Kay, for whom Saturn served as squire. I did more research for that RP than for almost anything else in my life. I busted my butt to make sure I had not just accurate references in the game, but the spirit of the earliest legends available as well. I learned just about everything I could possibly learn without reading the Triads in the original archaic Welsh. Now my partner is planning a trip to visit Aberystwyth University in Wales, the school providing her distance learning course that she'll start in the spring. I can't help but wonder if the game had anything to do with that decision.
Current projects: 
I can't resist the silliness of that world.