It's interesting how this topic of "the most difficult" piano compositions come up so often.It really depends on the pianist, doesn't it? Some describe Ravel's Ondine being devilishly difficult, but it was a breeze for me, though I struggled with Scarbo.However, one composer rarely mentioned is Alkan. His Etudes, particularly the minor etudes, Op. 39 are very taxing. His compositions make Liszt, Prokofiev and even Godowsky pale in comparison.