Hardest Instrument to Play?

I'm curious as to what people think is the hardest instrument. If you could list your answers in these categories ( To Learn, To Teach Yourself, To play, To Master, BONUS: To March, but only if you have played in a marching band before) that would be appreciated. I know people are all going to say their instrument is the hardest, but please try to be as non-biased as possible. Additionally, if you could list the instruments you play that would be great (you can also list instruments in the same family, but only if you have had experience playing both) I play the piano, flute, and piccolo. Of those I would say the piccolo is the hardest. I believe it is very difficult to learn (to get a sound), and even when you can play it, it sounds awful unless you are spot on. That is just my opinion, though, and I would like to see what everybody else thinks.