Rift, it was really fun at first, and it has probably the best customizability, but it got boring really quick; it was too much like WoW imho.
Edit:
I think the biggest problem I had with the game was the story. To the best of my knowledge both sides (Guardians and Defiants) want to save their world but the Guardians were chosen by the Gods and the Defiants weren't, so they use magic and machines to try to save the world... Yet both sides hate each other like (pardon the WoW reference) the Horde and Alliance, yet unlike the (another WoW reference, it's all I've got to go on) Horde and Alliance, at least up until Cata, can't put their differences aside when a larger threat is threatening to destroy their world. I just find that silly because they can't put their petty differences aside to stop everyone from being killed and/or zombified.