If you are talking about existing songs used in a movie, these don't qualify, but I have always thought the theme song for "Jaws" and "2001, A Space Odyssey" are the tops.
I know ridicule is coming, but "Somewhere Over the Rainbow" has to be one of the most recognizable songs tied to a movie.
[video=youtube;vVof0qj7SOw]http://www.youtube.com/watch?feature=player_detailpage&v=vVof0qj7SOw[/video]
[video=youtube;H5pnRW4wyRw]http://www.youtube.com/watch?feature=player_detailpage&v=H5pnRW4wyRw[/video]