The song that gets stuck in my head all the time--Actually I have a theory about songs running through my head. I have found sometimes that the songs that run through my head reflect how I am feeling in some way.
Even so, I get annoyed by the song eventually. My DH has a song that will always get an annoying song out of one's head. The Smurf Song. Of course then you have to get that one out. There's always the Song that Never Ends!