I would like to know and compare the most difficult pieces in everybody's repertoire.
I've gone through quite a few difficult pieces, and learning some more right now.
Difficult pieces in my repertoire:
all liszt's first 15 out of 19 hungarian raposidies (if you don't count the spanish raposidy)
liszt's paganini etudes #3, #5, 3 concert etudes #3, 2 concert etudes #2
liszt's transendental etudes #1, 2, 3, 4, 6, 7, 8, 11
liszt's sonata in b,
liszt's mephisto waltz
rachmaninoff concerto #2 mov 1 (can this be considered very difficult??)
rachmaninoff concerto #3 mov1, 2 (3rd mov in progress, can't polish up the octaves quite yet; can't really count the 2nd mov. because it needs the 3rd)
rachmaninoff sonata #2 (1931, when i first bought the music, i got that one, after getting used to it, can't really migrate to the 1913 version)
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pieces that i terribly want to learn, which i don't think will be possible:
brahms variations on a theme by paganini (both books, can only terribly get thru)
rachmaninoff sonata #1 (not quite famous, but certainly very difficult, haven't heard anyone mention this one for a long time)
ravel's scarbo (can't even play at the slowest speed)