Leopold Godowsky's technique was such that Arthur Rubinstein wrote, "It would take me 500 years to get a mechanism like [his]." Which came in handy playing his 53 Studies on Chopin's Etudes - which are often nominated as the most difficult pieces in the repertoire.
What's the most difficult piano piece? Opinions vary. Is it La Campanella, written by Liszt to show off what only he could do? (performance, score) Is it Balakirev's Islamey, which even Balakirev struggled to play? (performance, score) Or Ravel's Gaspard de la Nuit, written to top Islamey? (performance, score) Does Godowsky double his points by reconfiguring the already-difficult Chopin for the left hand? (performance) And if someone plays all four hours of Sorabji's Opus Clavicembalisticum, written across four staves to fit the extra notes, will anyone listen? (perfomance excerpts, score excerpts)