The Dance of Eternity, the 7th movement from Dream Theater's Metropolis Part 2: Scenes From A Memory
I think this is up there as one of the most technically challenging songs for not only a single musician, but a whole band to play. It's second on my list, The Black Page by Frank Zappa is probably the hardest.
https://open.spotify.com/track/2NmBV...S--3zrLLVyJoJg
I came across this song on Spotify today, and I was floored.