J.S. Bach
I would have liked to hear Beethoven play piano before his hearing went. I would have liked to hear him play his chamber music, especially to hear how he handled certain piano passages in the “Kreutzer” Violin Sonata (Op. 47) and the A Major Cello Sonata (Op. 69).
And the Bachettes. All 16 or so of them.
I missed the whole 70's Prog era, but I would like to go see ELP, especially to see someone like Mr. Speedy who was fluent in Classical and Jazz play on that somersaulting concert grand. There's a great YT video Oscar Peterson introducing Emo on his show with that clip.

👍
The Jesus of modern music in one sense from which all flows....no question
I loves me some Bach
Baroque not classical
A slight but important distinction...
Bach had some sort of immortal induced talent on loan from God sort of thing....