Carmen Fantasy by Borne for flute is quite hard, but it's mostly just a load of either chromatic runs or general scales. And a little bit of double tonguing here and there. For piano I think that L'alouette (lark) by Glinka is pretty amazing, and I hope to get to that level one day, though it looks ever so slightly impossible for someone with small hands!! Lastly, for violin I would say that one of the most difficult pieces could be Carmen Fantasy (again, I know!) by Sarasate, or maybe any of the virtuoso repertoire actually - after all, many of the great composers were virtuoso violinists themselves. Anyway, sorry about the longest post ever!!!
