Kaikhosru Shapurji Sorabji
Sorabji has arguably written the most difficult music ever performed.
He is mostly well known for the difficulty of his music. Most of it is written on 3-7 staves because he couldn't fit all of the notes on two. His largest works are anywhere from 3-10 hours long. His piece "Opus Clavicembalisticum" (commonly referred to as the OC) is considered by many to be the most difficult piece for piano ever performed and recorded.
Opus Clavicembalisticum is 250 pages long, and one can only see the sheetmusic to fully appreciate its difficulty. It is so incredibly difficult, that so far, only two pianists have released recordings - Geoffrey Douglas Madge, and John Ogdon. Unfortunately, both of these recordings are, for lack of a better word, terrible. Madge improvises most of the piece, and Ogdon's interpretation is so harsh that he makes most of the piece sound like noise. There is, however, one pianist who is currently performing the OC who can actually play it well - Jonathan Powell. He has not yet recorded it, though.
Some sheetmusic samples of the OC -
www.opusclavicembalisticum.blogspot.comRecording of Sorabji's shorter piece "In the Hothouse" -
https://www.sorabji.com/sound/piano/hothouse.ramTwo fascinating interviews with pianist Michael Habermann about Sorabji's music -
https://www.michaelhabermann.com/sounds/_index.htmlA topic I made about the many mistakes in Mr. Madge's CD -
https://www.pianostreet.com/smf/index.php/topic,12002.0.htmlAs for recordings to buy, I recommend the Fantasie Espagnole, Le Jardin Parfume, Gullistan, Djami, the 1st Sonata, and Michael Habermann's recording of Sorabji's transcriptions. If you enjoy those, you might want to look into the 4th Sonata and 1st Toccata.
I do not suggest that you buy either of the OC recordings. Wait for Powell's.
Here is a sample of what Sorabji's music looks like -



What's amazing is, despite its complexity, Sorabji wrote some of the most beautiful music yet written. I really would suggest looking into his music. I don't think you'll be disappointed.
Michael Finnissy
His music is incredibly difficult. Some of it is also very beautiful, but I'll tell you the truth, if you don't appreciate 20th century music, you probably won't like Finnissy.
Here are some samples -



What it sounds like -
https://fuwatm.hp.infoseek.co.jp/finnisy_english_country_tune002_choice.mp3Conlon Nancarrow
Nobody really knows how much of Nancarrow's music is actually possible for humans to play. His music is so difficult because of the extremely complex rhythms he uses. For instance, he'll often have different hands in difference time signatures, playing at the same time!

Xenakis -
Difficult to describe. He wrote solo piano music using up to 10 staves. He deserves a mention for sheer insanity.
Xenakis - EvryaliBest regards,
John Carey