Joking aside, MMOs require a larger time investment than any single-player romp. While claims of playing since a game's beta are almost a self-parody, there's a lot to be said for dedication to a single game, and it breeds far more resources for a player to make use of. On the flip side, of course, you can only spend time in so many different worlds, and having a breadth of different experiences is more to the tastes of some players.
So what MMO have you been playing the longest? Is it a game where your subscription has never lapsed, or one you've frequently left and come back to? Are you happy to keep playing it, or is it that crazy one that you keep almost breaking up with but never quite go the distance?