I think for me, it's "Sing", mainly because it seems like the one SST song that's been covered more than any other... even though that's not to say I don't like it.
Other than that, for a while there, I felt the way Oscarfan did, only for "Shake Your Rattle, And Roll" by Baby Fats Domino: likewise, it seemed like it was on PWMS ALL THE TIME.