For me, being an absolute MMO addict, I'd have to go with one of potentially three.
Either City of Heroes (repetitive, but the combinations last a while), Lord of the Rings Online (best community I think I've met to date, and the music playing feature is just pure win), and Champions Online, if they don't drop the ball on it.
Champions has both mix-and-match powers for incredible customization, with the powers themselves being able to be customized AGAIN after you take them, and enough costume part creativity to make City of Heroes cry in a corner. So as long as Bill Roper doesn't dig his dirty little hands into it and pull a Hellgate, that'd probably be the winner.