Interesting responses.
To be honest, I always prefer that my friends who come over and want to listen to my system, pick their own music. I have found that people tend to be more excited listening to music they know and love, rather than somebody else's favorites.
However, when a friend says "Impress me", I will typically throw on a 45 RPM single of Satchmo's "St. James Infirmary". Most people already know and like Satchmo, and this piece only reinforces their fondness for his music. It is very well recorded, and sounds fantastic.
My two cents worth.