I've played many of the big ones (expect Ultima Online) so here's my list:
EQ: LOTS of content. I've came back to it just recently and started a new account. It's a lot easier to make money and gain that next strong item (thanks to the player-ran bazaar). However, it's not the most solo-friendly game. You can play it solo, but many of the higher-level zones will be impossible for a solo player to hunt. Plus once you get into the 30's, the XP drag is a real pain. Couple the massive the amounts of XP needed to level with not being able to find a relatively safe place to hunt, you could be waiting a while to level. Beyond that, if you do group up (which EQ is designed around group-play) you can have a lot of fun and many of the quests are pretty fun.
City of Heroes: This is a really good game...for a while. After a while, the game's lack of any real difference in the way battles are fought can decrease the enjoyment. There isn't a lot of content right now, but I would assume that as long as the player base stays big enough that new and different content will be added to the game. The major plus to CoH is that it's fairly solo-friendly.
Dark Age of Camelot: This is a decent game, but I don't think that Mythic truly knows how to balance out the realms or even the various character classes. Solo play is possible, but many higher-level quests need a group to complete. DAoC is designed around PVP-realm combat. If you don't really want to pit yourself against other players, I steer clear of this one.
Horizons: Horrible. This is the worst MMORPG that I've ever played. It's clunky, unattractive, and simply not all that much fun. It's only real draw is the option to play as a dragon.
Anarchy Online: I only played this one for a little bit. What I played was OK, but I wasn't enthralled with the setting. If you want a futuristic game that plays like a fantasy game, this is the one for you.
Star Wars Galaxies: I was really psyched for this game, and it failed. Beyond the places and species, there is very little here that gives the true feel of Star Wars. It does have an interesting class format though. All in all there is nothing Star Wars like about wandering the wilds of Tatooine killing wildlife and harvesting the carcasses to make new gear. Oh yeah, becoming a Jedi requires completely a difficult quest (as well it should) and then it only unlocks a new Jedi Character for you. The character you complete the Jedi Quest doesn't get to be a Jedi though...kind of a bummer there.
Final Fantasy 11: This is hands down the prettiest game I've played. However, with all the innovative ways of handling magic the FF series has presented, they don't bring out any new ways of casting magic. This is another game that can get tedious very quickly if you are not in a group. Plus, finding a group among Japanese players as well can be a bit of a pain since the translation tool doesn't always do the best job. This is definantly a game where you need to keep up with your next level of gear as well.
Asheron's Call: Not sure if this one is even still active (although I'm sure the sequel is). This is one ugly game. That said, it's very solo-friendly. Just don't expect much to look at while you're playing.
There's the list. Hope that helped.
Kane