All I can tell you is that out of the Chopin pieces the ocean etude is probably the most difficult, and out of the Mozart pieces, the K. 533 is probably the most difficult (which I did not know at the time I selected it...sigh...). For some perspective: in Hinson's "Guide to the pianist's repertoire" he labels pieces as I (Intermediate), M-D (medium difficult), and D (Difficult). Most of the Mozart sonatas as well as other pieces including Chopin's ballade in g minor (strangely enough) he lists as M-D; pieces like Rachmaninoff sonatas, Chopin's ballade in f minor, etc. etc., are listed as D for difficult...as well as two of Mozart's sonatas: the last one, and K. 533.