"Riki Tiki Tavi" by Donovan in Barbarian

I won't spoil exactly how this track is used in the film but suffice to say it is brilliantly used as a foundation that ultimately bridges the gap between what feels like two very different stories. At once a palate cleanser and a sharp forewarning that one must solve their own problems, the song sets up the rest of the (very intense) movie.

Runner up goes to the inclusion of "Be My Baby" at the end of the film. One of the greatest songs ever recorded and if you've seen the movie — a great fit for other reasons.