Best?
1) Gemstone 3
2) Magestorm
3) Everquest (sort of)
4) City of Heroes
5) Eve
Worst?
1) Vanguard
2) Asherons Call 2
3) Age of Conan
4) Lineage 2
5) WoW
The way I rate this is by how well the game was designed in perspective of it's release and how much fun I had with it at the time. For example, I think as of right now Everquest is a horrible game, but when it first came out it was amazing for what it was. It kept me playing endless long hours, missing sleep to barely make it to work, and dreaming about it up until Scars came out. That is not to say I didn't see its faults nor was colorblind to many glaring bad game play aspects. Now you couldn't get me to go back to play that game at all though.
I will say that at this time, no MMO I consider all that good right now. None of them on the market. Right now I'm barely playing Allods and Alganon just for something to do, but they aren't all that great. I am refusing from now on to give my good money to badly developed games. Now the free games I can handle for now such as Allods or Penumbruum and betas.