I've always thought it kind of stupid that people consider 'death waltz' to be the hardest piano piece. The reality is that it doesn't have any technically tough passages or complex rhythms or anything, it's just that someone has doubled all the octaves and added a few useless fifths and thirds to make it have more notes.
People have been doing that kind of stuff ever since western music composition has existed. Ever noticed how some harpsichords have multiple keyboards? That's for doubling octaves and terracing dynamics. But do we consider organ partitas to be 'the hardest piano pieces'? Generally not.
If you want to hear a really difficult piano piece that doesn't just 'look hard' but actually has a great of technical difficulty, look at any of Liszt's transcendental etudes. I recommend #2, it's tons more complex than the 'death waltz' and as a bonus, it's physically possible to play.
https://www.youtube.com/watch?v=QiAVyMLtCH4
