I would probably choose Barber's adagio for strings. The biggest thing I think most people miss with classical is that they believe it's soulless and boring. Something slow, and emotionally resonant is the perfect piece to make someone challenge every belief they had prior, and to fall in love with what they hear. Adagio for strings is a perfect representation of that, I think. Technically simple enough that the emotions shine one hundred times brighter. It honestly makes time stand still for me whenever I hear it.
By - Sausage_fingies