To me, the hardest pieces are two-fold. The hardest technically I would say are Liszt's. They require great technique and an open mind. And very fast hands. I played La Campanella, and during rehersal, tore a tendon in my hand (which led me to work on my technique!).
On the other hand, i think that there is the interpretive stance. The hardest pieces to play well, to me, are Chopin. He music is so beautiful and intense, and technical, that it is hard to play it too fast or to try to show off. With Chopin, it's easy to want to show off, but it's dangerous because it's the very thing it's not intended for. So, his music is hard to play well.