The one who seduced you and fucked you over and broke your heart in a million pieces and laughed about it: Grey's Anatomy.
The old flame you don't see very often any more but whom you still really enjoy getting together with for a few drinks and maybe a pleasant nostalgic romp in the sheets: Doctor Who
The mysterious dark gothy one whom you used to sit up with talking until 3 a.m. at weird coffeehouses and with whom you were quite smitten until you realized he really was fucking crazy: Harry Potter.
The one you spent a whole weekend in bed with and who drank up all your liquor, and whom you'd still really like to fuck again although you're relieved he doesn't actually live in town: Firefly/Serenity, though if he were still around I'd still be screwing him.
The steady: Supernatural
The one you repeatedly cheat on your steady with: Stargate Atlantis
The alluring stranger whom you've flirted with at parties but have never gotten really serious with: Stargate SG-1
The one you hang out with and have vague fantasies about maybe having a thing with but ultimately you're just good buddies 'cause the friendship is there but the chemistry ain't: Queer As Folk
The one your friends keep introducing you to and who seems like a hell of a cool guy except it's never really gone anywhere: Bones
The one you slept with on the rebound who still smiles at you, yet you have no interest in any more: Lord of the Rings RPS
The one who's slept with all your friends, and you keep looking at him and thinking, "Him? How the hell did he land all these cool babes?": Twilight *shudders*
The one your friend has fallen for like a ton of bricks and whom she keeps babbling to you about on the phone for hours, and you'd be happy for her except you just know it's going to end badly: Twilight/Heroes
The only two I didn't get to pick for this were Buffy and Popslash. Popslash was like that awful first boyfriend who you're embarrassed you dated. Buffy/Angel was like my first love, I still miss that fandom.