I'd love to live in any game universe, so I guess for me this is just the "least favourite of [awesomeness_here] universes" sort of thread. Anyway, in that case it would be the World of Warcraft universe. Fine in general, but as soon as I decide to go raid the Alliance I get some idiot in my horde screaming randomness at the top of his voice and ignoring all my well laid battle plans. Much like the constant experiences I have on Halo multiplayer.
Besides, it's only a matter of time before gaming and technology develops to the point where we can literally live within game universes anyway when playing a game. Natal is just the beginning, you'll see. Our time will come...