I really liked DAoC. I think the classes in that game were the most varied and interesting. But DAoC was a grind game and a particularly bad grind. I played D&D online and liked that especially since it is free.
WoW is the game that kept my interest for the longest. I closed my account to finish up my degree. I may get back into it. From a game-playing aspect WoW is still a good un.
Briefly I played City of Heros and I understand it has gotten a lot better but I don't think I will go back to it. But a very interesting concept.
The issue that I have with online games is that I prefer to play solo, which kinds defeats the purpose.
It is too bad there is not a single player version of WoW for us anti-social players.
But until I gradumacate, I can't afford the time requirements for online games.