Dungeons and Dragons Online was the first. Played that for a year or two, enjoyed it before it became what all MMO's do after prolonged playtime: Fucking. Too. Repetitive.
Went to Guild Wars, really enjoyed that for nearly a year, before my guild fell apart. Log in every so often until Guild Wars 2 comes out. (^_^)
Then to Star Trek Online, got three Vice Admiral characters. And my favorite ship, the Sovereign class (Enterprise E). Currently taking a break from that, for a better part of this month due to school.
..That's about it.
Edit: No, wait, forgot Global Agenda as well. Haven't gone on for a little due to lack of a guild (corporation), though.