Anyone up for Kaikhosru Sorabji’s "Opus Clavicembalisticum"? It is about 4.5 hours long and almost every minute is exhausting to listen to because of the complexity and changing rhythm. I particularly like the last two sections of the work (the fugue and the coda section).
I also agree that the Nancarro works are amazing. They are not conventional piano works and cannot possibly be played by a human.