Yeah, WoW has the most players. They're around 9.5 million from the article I read a few weeks ago. Star Wars had 1.5 million when it was first introduced and quickly dropped to about 300,000. They're getting ready to go to a free to play, pay to play hybrid model. In fact MMO's in general are suffering with reduced player numbers over the last two or so years, because people are getting a bit bored with them I guess.
I just started Guild Wars 2, and it has a pretty good amount of players. We'll have to see if that stays that way after six months though. A lot of games lose their player based with in the first 12 months. Tera and Secret World are a good example. Both were introduced earlier this summer, and both now have about have the player base as when they started.
Eve is the longest running game with an active player base. They don't have the same numbers as WoW, but that game has been around for over a decade and it's still going strong.
Doesn't matter though. They're all fun to play if you're looking for a game it's more about getting along with people and certain aspects like how much you want to play vs how much you want to pay. That's why I decided to switch from DC Universe Online to GW2, because I didn't want to pay out a subscription each month for an online game anymore.