The HARDEST chopin etude.... (op. 10 no. 2 "Chromatique")
Sources: Josh Wright said he played full Rachmaninoff concertos and etudes like winter wind but this was still harder. Traum said this and op. 25 no. 6 were the hardest. When looking up the hardest Chopin etude, this, op. 10 no.1, and op. 25 no. 6 were most commonly mentioned. I found it hard to believe myself, because the notes seemed easy to learn but going from learning the notes to getting it up to tempo was like a logarithmic function.