I guess, it must be somewhere between Liszt's Apres un lecture du Dante, or La leggierezza, or Hungarian Rhapsodies 12 & 15, or Chopin's Etude Op.25 No.11 (winter wind), or Sonata in B minor, or Andante Spianato & Grand Polonaise Brillante, Op.22. Technical difficulties abound throughout these pieces.
Then there are other pieces that are very difficult in specific sections but not throughout the pieces, like Liszt's Il lamento and Chopin's Scherzos 1 & 3 . . .