EASIEST
Op. 10, No. 9 [This one is all just left hand extensions. Tons of left hand stuff.]
Op. 10, No. 12 [Again, this is an etude for the LH.]
Op. 25 No. 7 [This etude focuses not on virtuosity, but rather on producing a gorgeous sound, like 10/3. This one would be perfect if you're looking for something slow and beautiful.]
Op. 25 No. 2 [If, as indicated earlier, you're a right hand specialist, then this one would definitely be easiest, as it focuses almost entirely on the RH. However, for most people, I would assert with some reservation that this is the hardest]
HARDEST
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LEAST FUN (Disclaimer: Extremely Subjective; the opinions here are the opinions of bachopiev and not necessarily of pianostreet.com)
Op. 25 No. 2 [Though this one is fast, and virtuosic, it has never really seized me. I love every single one of Chopin's 24 etudes, but out of this bunch, this one grabs me the least.]
Op. 25 No. 7 [Heart-wrenching, gorgeous gorgeous etude. The fact that this does not make the top just goes to show how wonderful Chopin is. This one is a true delight to play -- not a delight in virtuosity, but a delight in profundity.]
Op. 10 No. 9 [An absolute delight to play. One of my favorite things about this etude is the delightful amount of rubato one can employ, and the liberty the pianist has when performing the etude.]
Op. 10 No. 12 [Absolutely incredible and exhilarating to play.]
MOST FUN