I was inspired to go through the etudes and mark how far along I was in each:
10/1 - Can do smoothly a bit below tempo. Not awful.
10/2 - Still experience finger jumbling and fatigue. Also, can't get dynamic control I want. Easily one of the hardest.
10/3 - Performed long ago, although I've gotten better at it since.
10/4 - Still get fingering problems I haven't worked out. Also one of the hardest
10/5 - Playable, and fun!
10/6 - I love this one. Never performed, but feel confident in it.
10/7 - Like many, I kind of neglect this one.
10/8 - Challenging, but playable.
10/9 - Performed years ago, but not to my liking. Shouldn't be that hard, but for some reason still gives me headaches trying to polish it.
10/10 - People say this is one of the hardest, but I don't think so. I find the different phrasings to be fun.
10/11 - A headache. One of the hardest.
10/12 - Performed. Awesome.
25/1 - I have to be relaxed and in the right headspace to get the feel right, but very satisfying when you do.
25/2 - Not hard. Got bored and neglected it.
25/3 - I still avoid this one. One of the hardest.
25/4 - I worked on it extensively, but still miss notes! Grrrr!
25/5 - Trickier than you think when you really try to get it performance-worthy, but I really enjoy it and don't find it terrible.
25/6 - I avoided this one for a long time, but was assigned it by a teacher and realized I can actually play it now. Still quite difficult, though.
25/7 - I still attest this is one of the hardest, if you really want to get the phrasing and control of the melodies right. Sure I can play through it, but I'm never satisfied with it.
25/8 - Performed. Fun and not that hard.
25/9 - Performed. Also fun.
25/10 - How seriously are you 'holding' those notes out in the middle of the octaves? I have to fake it a bit, because my hand just can't do it. Great piece for pounding out some aggression.
25/11 - Working on now. Actually playable! The RH falls into place easier than 10/2 IMO.
25/12 - Performed. Love it. If I'm not warmed up it can fall apart pretty easily, but once you get in the groove not one of the hardest IMO.