Know of or know how to play?
I expect that it would depend on the performer, but for piano I believe that the most difficult pieces are written by Ferneyhough. In my opinion they are by no means lovely to listen to, but they are certainly difficult. For pieces that are very difficult and yet retain a general sense of beauty, Rachmaninoff's 3rd Piano Concerto is incredible, and there are always Liszt's Transcendental Etudes...
The hardest piece that I can play would have to be one of my compositions. Probably the current idea of the third movement of my Piano Concerto.