I’m defining this not as the best song in a movie, but best use of a song at the beginning of a film to set the tone. It could be a song used as a cold opening, or one used at the end of the first scene which runs over the opening credits. Some examples:
Hmmm. I generally don’t remember the opening songs of the movies I’ve watched including those movies that I thoroughly enjoyed watching. I do remember the opening songs of movies like Star Trek and Terminator but that’s because I know/recognize their signature songs and they were played during the opening of the movie.
The ONLY movie whose opening song I can remember (other than the movies that played signature/theme songs that I know by heart, that is) is Lion King. I loved the movie when I saw it as a kid but now it’s not a movie that I would add to my DVD collection. But I still love the opening song (the one sung in the movie and not the one sung by that guy) so I guess that song would be the best song in a movie opening for me.