I picked this up after getting one of my first serious gaming rigs. DarkSpace was (and probably still is) a ton of fun, as you try to skirmish to conquer a star system with a team of players, opposing another team of players. Fight ships, build up planetary defenses and offenses... a simple concept, well executed.
The larger galactic conflict, a persistent universe of conquerable worlds, never really did catch on with the majority of players.
My first true conventional MMO, the cyberpunk fan of me couldn't resist the draw of Neocron. I spent my time as a "droner", remotely piloting my flying drone of doom in the post-apocalyptic wasteland, winding down the night by helping reintegrate resurrected players' implants in the downtown core.
PVP was fun, but there was almost literally no reward to the risk. Quite the opposite in fact, where the act of PVPing was an expense which forced you to PVE constantly to support the effort.
Kudos for style though, this game nailed the cyberpunk themes including its own red light district.
3. Final Fantasy XI Online
The promise of a Final Fantasy MMO made me take half a day off work to go pick this one up. It took almost literally the entire day and evening to install and patch this beast, so I didn't get to play it until the next day.
Great fun for the first 15 levels or so, then suddenly the solo game is 100% closed. You're forced to group, and at the time the Japanese player base had a year on the rest of us -- and wouldn't play with most international skill-less players. Probably one of the most hostile communities I've seen in an MMO, and instantly killed the game off for myself and my friends.
4. Earth & Beyond
Before EVE was out of their test phases, E&B beat them to market. A casual space MMO, it attempted to fill that niche with high production values, an evolving story, and clever team-based combat mechanics. Unfortunately the serious storyline was crippled by writing which never took itself seriously, and a publisher who pulled the plug on the game rather than support it.
5. EVE Online
By far my favorite MMO ever, a sandbox game where the player sets their own goals and attempts to thrive in a universe where the phrase "player-driven" is taken 100% seriously by the developers. At the time of this writing, the game has undergone revisions over the years to keep it well ahead of any other MMO on the market when it comes to technical prowess. The community is strong, the game continually supported by the the developers with regular, free expansions.
6. EverQuest II
EQ2 brought a lot of promise back when it first launched. The engine was way ahead of its time, so much so that turning its settings to maximum resulted in a slideshow. But having voice acting for NPCs and very high production values made this a very interesting game, which is still supported to this day with expansions.
7. Guild Wars
I always found this to be more of a massively single-player online game rather than a traditional MMO. With really gorgeous styled designs to the world and characters, and a fantastic soundtrack by Jeremy Soule, I always liked coming back to this game every six months or so to re-immerse myself in the story.
8. Dark Age of Camelot
One of my favorite MMOs ever, with enjoyable PVE and way-ahead-of-its-time PVP/RVR. There was nothing more fun at the time than sieging the keeps owned by one realm with a big group from yours, and getting ambushed by a third. The tangible realm-wide rewards to succeeding in RVR made it even sweeter.
9. Dark Age of Camelot: Darkness Rising
Great expansion to DAoC, really tightened up the leveling for players who wanted to make new characters and knew what they were doing. Added some lush new areas, lots of new NPCs and quests. Probably one of my favorite MMO expansions.
10. City of Heroes
I played this at launch and had a lot of fun with friends. Pretty much everyone would agree the character builder was a stroke of genius, leading to myself and my friends becoming altaholics. I stopped after the first major nerfing which decimated the ability for organized parties to attack challenging opponents. Being skilled was seen as being exploitative, and I would not continue to pay a monthly fee to fund developers who did not understand their own game.
11. World of Warcraft
WoW and I have a sordid history. I played it, like everyone, and actually enjoyed it for a time. But sooner or later I came to the realization that at level whatever-I-got-to, I was basically doing the same things against the same monsters, with the same abilities. The scenery changes, but that was basically about it. As addictive as it is, it's definitely not my favorite MMO simply because of how generic I found it to be.
12. The Matrix Online
This should have been an easy MMO to develop. The source material is pretty much limitless, and yet the ball was dropped in ways that few developers ever drop them. The promise of a persistent live storyline was ruined by the players, with ineffective moderation in place to manage them. The combat system flawed to the point that it was never able to be fixed properly. If anything, this game became a cautionary tale for others.
13. Dungeons & Dragons Online: Stormreach
This is one of the few MMOs I've seen where pickup groups seem to work. Everyone's class has a purpose, and was useful for the various encounters available at launch. I've not played the more recent free version of the game, however.
14. Star Wars: Galaxies - An Empire Divided
Oh god. What more can be said about this game's history that hasn't already been said. I played it after the NGE happened, and the experience was enough for me to not bother with this for more than the first free month of play. Once again, an awesome pile of source material wasted. Lets hope the Bioware Star Wars MMO fares better.
15. Richard Garriott's Tabula Rasa
Conceptually was interesting, and I really enjoyed the story behind the game. The mechanics of the real time combat didn't quite live up to my expectations, and the fact it was basically a quest grind ala World of Warcraft made me walk away from this one.
16. Warhammer Online: Age of Reckoning
I'm blown away this game isn't extremely successful. It started out well, almost everyone played it. Public quests were a stroke of genius, and the three-way realm vs realm combat was an incredible evolution of the gameplay from Dark Age of Camelot. Somehow, confusingly, everyone stopped playing a month or two in.
17. Age of Conan: Hyborian Adventures
Broken at launch, bad game play mechanics across the board, antagonistic PVP. Perfect combination to destroy your game's base of subscriptions almost immediately after release.
18. Star Trek Online
Played for a short time at launch. Far too many bad design choices on the part of the developer and publisher to stick with this one.
Update 1: I've resubscribed to STO for another month or two to see how the latest content updates have fared. I've restarted my character to experience it fresh again, and so far things are a bit improved on the content angle. Still some game breaking bugs though (ATI cards crash when dynamic light's used on ground missions).
19. The Lord of the Rings Online: Shadows of Angmar
Incredible use of the lore of Lord of the Rings. Definitely looking forward to playing through the content of this, and the expansions.
After trying out the beta and being utterly blown away by the level of polish on this title, I snagged it at launch. So far my impressions are that Rift has an interesting balance of MMO staples with some awesome new concepts. Definitely a must-try for anyone liking something different from the usual high fantasy MMOs out there.