Well, if we're counting first person shooters in this category:
1) Ultima Online
2) Star Wars Galaxies (Pre Combat Upgrade, experienced through the emulator project)
4) Star Trek Online
5) Planetside 2/Warframe
I wouldn't normally count Warframe in an MMO list, because it's 4 players per game at most. Yes, it has a lot of people playing it, a lot of people to potentially match up with, but as far as I know, it's only in 4 person teams. I don't really consider 4 person teams to be 'massive multiplayer'. Not when battlefield 1942 had about 10 times as many players on a map, or even per team on some servers. That's why planetside 2 takes my number 5 spot. Because when you can have 2000 people on a single continent, divided into 3 teams, That's massive multiplayer. This isn't to say warframe is a bad game, far from it, it just doesn't seem to fit in the massive multiplayer category to me.
As for the top games on my list, there seems to be a lack of experience in this thread with regards to older MMO's. If you can stand to play a game with sprite graphics and a very different type of user interface than mmo's currently use, I'd suggest checking out a free shard for Ultima Online (That is, a free to play, private server that, due to a loophole in the laws, is allowed to exist for ultima online when it isn't for other mmo's). I would suggest ABCUO, since it has one of the more forgiving rulesets for players. I'm pretty sure a quick google search for abcuo will get you the right site.
If you can stand to play a game that came out around the same time was world of warcraft, you might have fun with star wars galaxies. It's not going to give you a storyline to follow, the fun in the game is making your own story and playing with the tools you have been given (or must build yourself). Galaxies is currently dead (officially) but the emulator project has a play server up if you want to try your hand at it. Like UO, you can do a lot of things in galaxies if you set your mind to it, including but not limited to building your own house, crafting sweet gear, and stabbing that guy for looking at you wrong in the bar. Unlike UO, galaxies introduced a mechanic that allowed players to establish their own cities (complete with mayors and shit). Unlike Ultima Online (the official one), galaxies did not feature a team of game masters who would log in each week and run in character events for the players to participate in.
For the more recent games on that list:
Star Trek Online has terrible ground combat. Seriously, it's awful. But you can have very enjoyable, operatic space battles in that game, and that's what makes it awesome.
Rift is like world of warcraft, but done better. By a lot. It has a few innovations in it, but a lot of the features should be familiar with anyone who has played an MMO. They are simply done better in Rift than I've seen in most MMO's. Helping that out is the fact that your class does not define what your character can do, but more how they do it. This means that a mage can tank, heal, dps, or support, so can a cleric, rogue, or warrior. The Mage just happens to use magic to do it, while the rogue...I dunno, throws powders or something. I don't play rogues in rift.
And Planetside 2 has massive combat. The only area it kind of screws the player over with in regards to basic gear is air combat. If you've seen angry joe's review of the game, that covers it pretty well. The point I'd make though is: this game works really well for mass combat. Stop worrying about getting some new gun or rockets on your fighter plane and enjoy the fact that you can go stomping into combat as a mecha-man and rock someone's face with your akimbo guns. It probably helps to think of planetside 2 as battlefield 1942, bigger, crazier, and uncut, instead of comparing it to more recent first person shooters.