My all time favorite comes immediately to mind; Daphnis et Chloe, the dance finale. I'm convinced that Maurice Ravel had his heart broken by a clarinet player because only that kind of treatment could warrant the part he wrote for clarinet (and flute and a few ither woodwinds) in the finale movement from his score to Daphnis et Chloe.
Things really his stride for the poor bastard at around four minutes in. If you look carefully, you can see his instrument burst into flames as if reentering the atmosphere at mach 12. The result is one of the most dramatic and emotional moments in all of music, imho.
The most difficult piece I ever attempted (that wasn't basically atonal) was Liszt’s Transcendental Etude No.4. I discovered quickly that this piece is outside of my capability completely. Here's piano superhero Boris Berezovsky attempting the piece:
The piece is so difficult that, upon performing the piece in public, inevitably at least one virgin conception occurs.