Chaconne by Johann Sebastian Bach, transcribed for piano by Ferruccio Busoni, performed by Alicia de Larrocha. It was originally the final movement of Bach's Partita No. 2 in D minor for solo violin.
Someone once observed that the Chaconne was spiritually complete, in that a musician could practice it alone for a year, and not feel impoverished. I could easily believe that.
The Chaconne is the most wonderful, unfathomable piece of music. On one stave, for a small instrument, the man writes a whole world of the deepest thoughts and most powerful feelings. If I imagined that I could have created, even conceived the piece, I am quite certain that the excess of excitement and earth-shattering experience would have driven me out of my mind.
--Johannes Brahms, in a letter to Clara Schumann
[It's] not just one of the greatest pieces of music ever written, but one of the greatest achievements of any man in history. It's a spiritually powerful piece, emotionally powerful, structurally perfect.