As someone who took (classical) piano lessons as a kid, I was interested in some online discussions I found about the most difficult pieces to play.
Here are two, and yep, they look damn difficult:
Feux Follets (Transcendental Étude No. 5) by Liszt, performed by Evgeny Kissin
Piano Concerto No. 2 by Bartók, performed by Yuja Wang
And the one for fun - Liberace plays an underappreciated "classic"
Liberace
Difficult Classical Piano pieces - and one for fun
Difficult Classical Piano pieces - and one for fun
Quidquid latine dictum sit, altum viditur. (Anything is more impressive if you say it in Latin)