Following in the footsteps of Cameron, who got the idea from F13 (which I don't follow regularly either), here's a list of my favorite MMOs. Only six, because these are all I care to mention.
1. Star Wars: Galaxies -- People trash it all the time, but, in its original form, SWG was a phenomenal game. It did many things wrong, but it pushed the genre forward as well (at least, it would have, if anyone had bothered to follow).
Visual customization was extensive and mattered. NPCs and creatures were active when not engaged in combat. The world felt alive. Characters had limited (mortal) health and strength, making them feel like true members of the gameworld, rather than outsiders who could crush NPCs on a whim and had no relation to them. It also resulted in tremendous diversity in character apparel and helped take the focus off optimization. The crafting system allowed more customization and experimentation than anything other to date. The taming of wild creatures opened up a whole new avenue of exploration. The Galactic Civil War was fun and evident everywhere, even emerging in the occasional NPC-vs-NPC battle which players could walk by or join as they please. There were many factions. There was true wilderness and a feeling of distance.
It was easily my most immersive MMO experience. I'm sure that was at least partially due to the fact that it was Star Wars.
2. Everquest -- My first MMO experience and, therefore, my longest (almost a year). Newbie content and advanced content were not initially separated by hard barriers. Level 10 and level 40 creatures often existed in the same zone, resulting in a real sense of danger and anticipation. Players were often surprised by encounters they didn't expect, which are generally more memorable than predictable encounters. The "DING!" acted as a bold celebration whenever the player progressed, cheering the player on. The world was huge and offered long exploration.
3. Everquest 2 -- A cinematic experience that I plan on returning to next time I upgrade my computer. Put the graphics on High, turn on the Letterbox, tap F10 to turn off the HUD, and the immersion is wonderful. SOE did a great job on the animations, which make the combat infinitely more interesting. Skills can be upgraded through exploration (loot drops). On High graphics, some creatures are camouflaged (like lions in the tall grass of the Commonlands). The best quests I've ever seen in an MMO were in EQ2, though they certainly didn't represent the majority of EQ2 quests. The world was more rigidly structured than SWG's, but still alive with non-combat activity.
4. World of Warcraft -- Finally, a decent combat pace in an MMO. The warrior's bloodlust, in particular, was a great idea, since it encourages players to move quickly between encounters. The art is imaginative and polished, and the graphics are good while also easy on computers. Some skill choice is available (though I was hugely disappointed that they didn't incorporate the full freedom of Diablo 2's system).
5. City of Heroes -- Being able to customize my character visually right away was great. More impressive were the skill customizations. Two players could have the same skill but have it tweaked different (greater power, faster retime, greater accuracy, etc). Chasing down fleeing in this game actually felt fun, since it often involved leaping over obstacles, climbing ladders, and pushing past civilians. Falling did damage to enemies and players alike, so it could be used tactically during combat on rooftops.
6. Shadowbane -- Originally, I was going to list my top five MMOs, but I have to list Shadowbane. The runestone system in character customization was a great idea. The skill system allowed for weapon specialization without reversal being impossible. It was my first PvP MMO and, while I wasn't interested very long, it provided some memorable PvP experiences. The newspaper or journal of happenings in the world and maps were good ideas.
Honestly, I'm more into single-player games and co-op games these days, but I'm looking forward to trying The Agency and Warhammer Online. Sadly, Trials of Ascension never left development and Darkfall doesn't grab my eye like it used to.