a few thoughts
1. I agree with Koji, the Ondine is definitely harder than Scarbo....believe it or not. I'm learning this piece
2. Why do people say Islamey? Yes, it is difficult. It is comparible to La Campanella, but both of these are difficult short pieces. You might as well put Feux Follet up there if you put these two up there.
3. I would rank Gaspard de la Nuit as one of the hardest in common repitoire, as well as the Liszt Sonata, and I agree with Paul, the Rachmaninoff Rhapsody is incredibly tricky, deseptively difficult. There are tons of trick bags in that (i'm also in the process of learning this).
On a tier higher than this, I would put the Busoni Concerto and Barber Sonata. These are just INSANE.