Alright, here we go in chronological order. <deep breath>
Asheron's Call 2, 7 months
Everquest, 6 months
City of Heroes, 7 months
Ragnarok Online, maybe 2.5 months (I know it barely counts, don't look at me like that)
World of Warcraft, six years
Warhammer Online, one week (most cutting disappointment of my life)
Age of Conan, two months (awesome concept, mediocre game)
Aion, one month (see above)
And like, 10 free mmos from Korea that weren't even worth that monthly fee.
... I had a girlfriend once, I swear. >>