Hey ho - it's that "hardest piece" thing rearing its uinwarranted, unwelcome and largely uninvited head here yet again! Yes, of couse, there are pieces like the wonderful Feux-Follets that are admittedly, by most people's standards, difficult to bring off as they should be, but in the end we are surely in something of a self-defeating situation here, to the extent that what might be difficult for some pianists may be less so for others, just as in the work of Alkan, Sorabji, Godowsky, Rakhmaninov, Skryabin, Medtner and/or any other major piano composer that anyone may or may not care to name and, when added to this consideration is that of the vast array of different difficulties to be encountered in piano repertoire of one kind and another, one may as well debate the question of whether Evryali or Song 9 are "harder" than Chopin's F minor Ballade or Alkan's Concerto...
Best,
Alistair