Most Difficult Piano Piece To Learn And Play?

A common question that is asked by piano player after they learn piano for a period of time is "what is the most difficult piano piece to learn and play?"

The answer varies.

A composer can compose a song with tons of notes to play. Sometimes, the notes can be all over the places (with more than 13-15 notes finger span); it also comes with impossible rhythmic expression (syncopated rhythm such as 5/8 beats or 3/5 beats, etc). You get my point.
I can compose a challenging piece even a music professor find it hard to sight read.

Personally, I think Scriabin probably wrote some of the most technical pieces in classical music. His etudes are rather technical and hard to master.

Some says Liszt's pieces are hard to master as well. I generally agree with this because most of us have small hands and hard to stretch more than 10 notes apart. Many of his pieces are written to fit his wide finger span.

Here is a Scriabin's etude played by Horowitz: