it depends on what you actually want in an MMO, i can say i have tried a ton of games that just doesnt hold up anymore! i can say about the only game that caught my attention for awhile was Maplestory, and that was mainly because all my friends were playing it back in Middle/High School when the game just came out! now-a-days i know some who still play it, and from time to time i play it, but not as i use too.
Another game that caught my attention that is recent was Marvel Heroes. granted im a huge marvel fan so it might sound a bit biased, but its a fun game. the only downside i have is that it seems like a game to play best with other people which sadly isnt for my friends, but i still play it. the main story is cool seeing villains and usually playing as the heroes of marvel's comic, although they have been releasing some villains as well (eg Magneto) but it has a large pool of heroes to play as, and its pretty fun to play with in general!