Which composer wrote the hardest études?

[chopineater]

I’ve been wondering which composer(s) wrote the most difficult études for piano? Which composer’s etudes are considered the pinnacle of piano technique?

[dizzyfingers]

I guess you haven't been browsing the "Recent Posts" list much over the past few weeks, you would have seen this very question on the list quite a few times.  It covers the hardest etudes with a lot of input from multiple people.

https://www.pianostreet.com/smf/index.php?topic=70967.0

[liszt-and-the-galops]

We have a pretty long-running thread for this: https://www.pianostreet.com/smf/index.php?topic=70967.50

Though I suppose I'll throw in my answers...
Hardest set of etudes that I'm aware of: Sorabji - 100 Transcendental Studies
Hardest set of etudes pre-1910: Mereaux - Op. 63 60 Grandes Etudes
Hardest set of etudes by a real composer pre-1910: Alkan - Op. 39 Douze Etudes dans tous les Tonnes Mineurs

Hardest individual etude that has been recorded accurately and at-tempo: either Ligeti no. 14 or one of Sorabji's
Hardest ... written pre-1910: Alkan - Op. 39 no. 8
Hardest individual etude, including impossible pieces, excluding random Musescore stuff etc.: Alkan - Op. 76 no. 2. Honorable mentions to Ligeti - Etude no. 14a and Mereaux - Op. 63 no. 45, which are possible but not at-tempo.

...Though I find the question of "best" etudes far more interesting.