It's always baffled me why people think 25/4 is hard. It just seems like some minor LH leaps and the usual musical difficulty.
Butterfly is on the easier side, but still the hardest of those three IMO.
10/6?!?! I thought that was agreed almost universally to be the easiest? (It's certainly the only one I can somewhat handle.)
The other ones you said made sense, but no Winter Wind?
Oops, I meant Op 10 no 7, of course! Looks like I also wrote Op 25 no 9 instead of Op 25 no 8. Clearly you shouldn't post when you are 5 minutes from going to bed. Edited!
Winter Wind is hard, sure, but I have had more trouble with other etudes. Maybe the technique in it just comes easier to me. The majority of both sets are quite hard but do not stand out as particularly extra hard, I think Winter Wind belongs among those (alongside for example op 10 no 4, 5, 8, 10, 12, and op 25 no 5, 9, 10, 12)
I think 25 no 4 is hard if you go for performance tempo in terms of
1) Accuracy on all the jumps. Accuracy is not the most important thing in the world for me but in this Etude it's kind of the point.
2) Keeping up the relaxation through the unrelenting series of leaps so you don't feel any tension or fatigue. If you got very good technique this isn't a problem but if there are any issues you can definitely struggle with this.