Godowsky's studies on Chopin,maybe.
Sorabij:100 trascendental studies are very difficult,
Cziffra transcriptions(from Strauss' Trisch Trasch polka),
Horowitz transcriptions(Carmen variations,Star and stripes   forever
available on Pellisorius Editions)
Brahms(Paganini Variations)
I don't know.
The difficult is all in your mind,I think...

"Barney Gumble" <[log in to unmask]>