Personally, my most played MMO is "FlyFF". It's old, and has nothing to do aside from grind/pvp, but it probably has the most freedom out of all the other MMO's.
Aside from that, there is also "Vindictus" which is a high-end hack and slash MMO built on the Source Engine. Pretty good and fun.
And if you want something a little different, theres also "All Points Bulletin," which is a GTA style MMOFPS.