I'm surprised that Brahms' Paganini Variations only got one mention in this thread. Some of the Variations should be unplayable, but apparantly every once in a great while a pianist who can actually play them surfaces. Arrau was one. It doesn't look too daunting on paper, but the tempo kills. And when one fine-reads it, there are 9 to 4's, fast parallels of every variation (parallel sixths in the left hand being a special evilness), outer trills etc. etc. Even when I hear it, I find it difficult to believe that var. 14, book 1 and var. 10, book 2, are actually playable.
Though I am new to this forum (Hi, how are you?) I sincerely doubt anybody here can play them. But then, Julius Katchen might lurking around here, so I could be wrong...