Lagrima by Francisco Tarrega is one of my all-time favorite classical guitar pieces. It is very simple in its design (ABA). The keys are E major going to E minor and back to E major.
Here are a couple videos I made back in 2008 of me playing Lagrima:
And of course, life would not be complete without a techno version:
scores, tabs, and mp3s available at http://www.robertlunncomposer.com