Every pianist will have their own challenges technically with pieces but more importantly, difficulty should also be measured on musicianship but is often not. Usually the technique is challenged first because you can't express yourself musically with out getting hold of the notes.
Alkan's music is difficult but many wouldn't consider it the most challenging. It more considered a waste of time as there is little music value in it however the works for L.H. he wrote are ridiculously hard an almost absurd in a way.
Liszt's music is quite accessible. I don't know why people think his music is difficult. Remember, he is a pianist and all his music is composed in a way that pianist can accomplish even the most difficult passages with practice. Every pattern fits in the hand and nothing is awkward. If pianists have trouble with Liszt, it means they have trouble playing scales, chords and arpeggios. The Transcendental Etudes have challenges but are not the hardest pieces. I've studied all of them and I have played much more difficult pieces.
The consensus amongst the performing pianists for most difficult pieces include
Islamay - An Oriental Fantasy by Balakierve
Gaspard de la nuit by Maurice Ravel
Petrushka Transcription by Stravinsky
A complete performance of all the Chopin Etudes
Often neglected Rachmaninoff's 2nd piano sonata 1913 edition. The 1931 edition was simplified by Rach but is still terribly difficult. The 1940 Horowitz version combining the 1913 and 1930 edition is ridiculous as well.
Rachmaninoff 3rd piano concerto will always rank as one of the hardest concertos.
Bartok Sonata for Two Piano and Percussion is one of the most difficult chamber pieces for piano ever.