This is a very tough decision, part of me wants to choose Star Wars, the other part wants to choose MLP:FIM
Hmmm.... Event though I like many of those games/series/books/whatever I'm not sure if I'd like to live in those worlds as many of them are hellholes. Constant warfare and threat of death, or even the extinction of your species. Some of those worlds don't even have indoor plumbing. Then again, even though Star trek is mostly an utopia I think choosing it would be taking the easy mode. (Especially since with the holodeck I could play in any of those other realities without the danger or other downsides.)
Hmm... Maybe One piece just because I like the utter insanity and awesomeness included.