Chopin's etudes are all difficult, but if you need to know which are the most difficult you need to look at two aspects here: the technical difficulty, and the interpertational/emotional difficulty. Chopin's etudes were the first etudes that demanded more than technical difficulty, it demanded musical and emotional mastery to execute right. Most of the etudes are technically demanding, but some etudes, the main idea of them is to put the technicallity aside and focus about musicality. So in my opinion, the three most difficult etudes are:
1. op.10 no. 11 - the musical main harmony element is REALLY hard to be played right.
2. op. 10 no. 2 - this etude is more technically demanding, but it's idea is that when you overcome the technical challenge, you need to play the chromatics so smooth, that it sounds like a much simpler piece (like most of Chopin's music of course).
3. op. 25. no. 6 - the double thirds are really hard to play, the technical element is really hard.
of course there are more difficult pieces, but as a piano player that player almost all of the etudes, this is my thought about it. I really think that when you play Chopin's music, you encounter some rough parts at the start of the learning process, and your fingers sometimes asks you if you are seriously playing this, but after you master it, the real difficulty begins, the struggle for playing a piece you master technically and putting in it the right amount of emotion, not too much and not to little.
happy etude playing, though
