Barber's Adagio for Strings was also used in the Platoon soundtrack from 1986. I would add to the list Albinoni's Adagio, the version from Gary Karr on King Super Analogue is superb and very melancholy. If that's your thing of course. I also like various versions of the Ave Maria, and of course, one must add Beethoven's Moonlight Sonata.