The one who seduced you and fucked you over and broke your heart in a million pieces and laughed about it: Lord of the Rings. And later, Harry Potter. :)
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: Band of Brothers
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:
Lost
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: Supernatural
The steady: Prison Break
The alluring stranger whom you've flirted with at parties but have never gotten really serious with: Numb3rs
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: Stargate Atlantis
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: The Office
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?": Ugly Betty
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: 24
The one who gave you the best damned summer of your life and who you measure all other potential partners against: Heroes
The ones you repeatedly cheat on your steady with: Battlestar Galactica, Grey's Anatomy, House, Dexter