https://www.pianostreet.com/smf/index.php?topic=462.0https://www.pianostreet.com/smf/index.php?topic=3255.msg29546#msg29546etc., etc.
It depends on how you define "hardest". For example:Chopin Op 10 no 2 is one of the most technically challenging pieces I have played in the sense that I have worked on it, on and off, since at least 2016, but still haven't "mastered" it. The most difficult bits I still need to push some 5-10 bpm before I can play the whole piece in Chopin's tempo of 144 bpm. It also still highly depends on my daily form if I can play it without mistakes or if I crash and burn at some point, so I certainly couldn't play it to an audience. Yet if I am in good form, it doesn't necessarily feel "hard" to play, it just takes some concentration.Another piece I find highly challenging is Franck - Prelude, Aria et Final, which is the lesser known but equally fabulous cousin to his famous Prelude, Chorale et Fugue. There's nothing in it that per se is astonishingly challenging from a technical point of view, but it's somehow just so massive and unwieldy and the Finale is relentless. It's surprisingly hard to completely memorize, which is funny because I rarely struggle with memorization and memorize things that some people find challenging like Bach fugues without issue. It's also difficult to hold together and make musically convincing rendition of, but I think it's a real treasure if you get it right. I have worked on it on and off since 2019 but still don't feel on top of it, neither technically, memory-wise, or musically. For comparison, the Prelude, Chorale and Fugue feels quite all right.
I have a lot of respect for those who can pull off Chopin op 10 no 2. That etude is HARD to play at tempo. I would be more scared of playing that than op 10 no 1, which seems to sit okayishly well in a normally sized hand.