Well, there's a huge amount to choose from isn't there?
For my own fancy, it's GW2 and Star Wars: The old republic. I've had the oppertunity to test out Star wars, but Guild wars 2 is looking pretty neat too.
Basically the main differences between them is that GW2 is trying something new with their quest system and the other advantage is that it doesn't have a subscription fee. Admittely tho, I am a bit tired of the fantasy genre.
Star wars is very classical type mmo, but it does improve alot on the formula, however it is subscrition fee based.
Then there's EVE online, WoW, Warhammer online etc etc. There's just a myriad of them out tehre