Hardest piece of classical music ever written?

[chopinlover01]

For this, I'm limiting it to music written in the baroque, classical, romantic, and 20th century era.
All composers are considered, and this is just for piano as always
Thanks everyone!

[visitor]

I think marble is probably the hardest I know of, but maybe there's another type of stone or stone tile that has been used

[cabbynum]

Franz liszt made a blog post the other day about how he wrote a new Hungarian rhapsody on diamond... His wins I believe

[thalbergmad]

A hardest thread, how unique.

[pianist1976]

Me thinks it's an estudie by Fridereque Chopinovski but i'm not sure

[lecafe88]

Mozart Sonata in C major K545. The C major scales are absolutely dreadful, far worse than the tremolos in Vers La Flamme or the octave glissando in Waldstein.