Lol, nobody mentioning EverQuest?!
I played EQ for a really short bit of time years ago (my lame old computer couldn't handle the graphics for long) but I absolutely loved it and would really love to go back to EQ or try out EQ2. I'd like to try WoW too - I have it, but it didn't work (I have awful luck there) and I can't afford to pay for it right now.
Dream of Mirror Online is pretty good, though I'll never play that seriously until they fix some certain... err... discriminatory things that are very applicable to me. The graphics are cute and it was pretty fun, very easy to figure out too.
I've played Furcadia for well over four years, but I'm not sure that counts as a real MMORPG. It's much more of a social game, though there are some absolutely amazing RPG dreams people have made and some lovely (if a bit abrasive at times) pokemon communities on it.
I have to say the worst MMORPG I ever played was Pirates of the Caribbean Online. Just. No. XD