With Etudes, it's a really tough call, because your previous experience is going to play a much bigger factor, seeing as how they test specific techniques. However, I think the ones typically considered the most difficult are the ones that have already been stated; it seems as though you have a pretty unequivocal answer. 10-2, 25-6 and 25-11 are usually considered the hardest. 10-1, 10-4, 10-10 and 25-8 are the other ones that will usually show up on these lists, but less consistently. 25-10 might pop up now and then, but I personally disagree with that choice. 10-8 is one that rarely shows up on these lists, but one I personally find very tough; more-so than 10-1 and 10-4, at least, but that's mostly because I'm left-handed.