For classical, I'd probably take Mendelssohn's violin concerto.
For pop music, whoever it was who mentioned kd lang's "Constant Craving" gets a high five! That song has aged well. I would probably have to pick something very melancholy, like Icehouse's "Angel Street," or a-ha's "Early Morning," or Leonard Cohen's "Waiting for the Miracle."
But really, just one? Why not ask me to choose which limb I'd prefer!