Depending on how "into" it you wanna get.
I would highly recommend DAoC (Dark Age of Camelot).
I've tried several different MMO's and to be honest, DAoC is one of the best I've ever come across. Like was stated earlier, the PVE isn't stunningly amazing, but there is ALOT of it out there and the RvR (PVP) is completely in a class of its own. Nothing else can even really come close.
This requires quite a commitment though, just FYI. Guild wars is a good middle ground for little commitment, high reward.
Kelemit