For me it would have to be Liszt's Hungarian Rhapsody No. 12 in C# minor. I started learning sophomore year in high school and finished in by the end of my senior year. What a year! Defenitely my favorite piece and most difficult that I can play. Very emotional piece as well, so it's very straining on my mind after I hit the last chord with a fortissimo.
EDIT: It's for piano
