Awesome, a bach thread. We can sit around and agree.
For me its the solo violin. Hes a polyphonic composer working with four strings. Theres some pieces he wrote for violin that have three notes at once. Turns out in those cases a note is virtual ‘play as if that note is there’..
https://youtu.be/UaMoPi9BOoQ
Not many people can play the music of that piece.