Samuel Barber's Adagio for Strings may be America's greatest contribution to the world of classical music. The point can be argued and discussed so feel free.
What is unique about the Adagio is that it's passion is tinged with melancholy from beginning to end. I cannot think of another piece of music that is filled with such resigned yearning, perhaps Mahler's adagietto from his 5th Symphony comes close. The Mahler was conducted by Leonard Bernstein at JFK's funeral.
In this Adagio, it is almost as if Barber were setting Michelangelo's Pieta to music. The face of Mary, as she holds the body of her slain son, shows a resignation to God's perfect choice. I hear that in Barber's music. This is a music that can guide us and teach us and remind us that our lives can contain some sense of the holy.