Beethoven's 5th on a Banjo. Perhaps one of the most well known classical pieces and is such a blast to play on banjo. It has some wonderfully challenging parts.
Please remember that I am no trained classical musician. I play the melodies and then add my own flavor, style, and improv. Improvising or making music up fresh, new, and on the spot is one of my favorite things to do. It was also a favorite of many of the great composers as it puts the player to the test to be flawless in the moment.