For me, it's Sigur Ros' "Svefn-G-Englar". I listened to that song about 1000 times after this one particular break-up (which also happens to be the last breakup I'll ever have to go through), and now I can't ever listen to it again.
For one person who posted at Stereogum: well, let me just control-V:
"The One Thing" by INXS... granted, it's not the greatest song, but it WAS my favorite song (that wasn't by Prince) when I was 14.
But that was all ruined a year or two later when I discovered a video tape of MY MOTHER STRIPPING TO THE SONG.