Tabula Rasa was my first mmo, but that was discontinued shortly after I started playing it.
Star Trek Online is the one I play most, but given the lack of the specific updates I wanted, I only login in rarely now.
Champions Online I have a permanent account for, but that game is not being updated at all, so I don't play it much.
Up until a few days ago, I was a full time player of Marvel Heroes.
But the flawed implementation of their latest playable character (War Machine) caused me to lose my confidence in the game, so I quit and am not returning until the flaws are corrected.
FYI, it is highly unlikely that the flaws will be corrected.
Looking forward, the number one mmo on my wishlist is a Fallout mmo.
Nothing concrete has been nailed down, but given that Bethesda authorized a Elder Scrolls mmo, it would be logical to expect their other major IP gets one.
Number two is a Mass Effect mmo.
But from player reviews, the Star Wars mmo didn't go so well, so that may be a concern with their next mmo.