Diku MUDs - first experience with morpg, lots of flexibility since text based. Massive amounts of original contents across various games, updated regularly.
Everquest - dabbled at a friends house, too much like Diku MUD (but less flexible) and I had to pay for it. Florida's rave scene exploded too busy to play this game
Anarchy Online - played off and on for 5 years. Typical first MMO romance. It was like most MMOs of the time, complicated and with a higher learning curve. Later dumbed downed through expansions. 2 servers to rule the world. The international component of this game is what really made it shine for me, there was always some nationality of people online so there was always options no matter when you logged on.
Matrix Online - beta tested this one. Personally what I think hurt it was the license, being coupled with the Matrix leads to certain expectations. A generic cyberpunk world would have done this game a lot of good and allowed the devs more flexibility in their design.
World of Warcraft - played with my friends off and on for 5 years. I always dislike it's heavy use of lockouts as it forces players to schedule time (I like raiding but at least in AO raiding was a more spontaneous affair). Extremely limited game, one raid path, gotten worse over time. I think this particular genre has devolved from Diku MUD.
EVE Online - Now this game is different, if you liked games like Elite or Tradewars this is the game for you. Personally it's too slow for me. Love the music.
City of Heroes - fun in small doses, by level 20 I had gotten most out of the game as I imagined.