Judging by your choices, I assume you are including only certain periods of music when you refer to "classical" music (baroque, classical, romantic and impressionistic). If no one beyond the impressionistic period is considered, it still remains a very tough question. I love DeBussey, Chopin, Medelssohn, Ravel, Grieg and Copeland. Also, Musorgsky is highly underrated. However, in the final analysis, there can be only one choice that stands out head-and-shoulders above the rest....
Ludwig von Beethoven. His 3rd, 5th and 9th symphonies are beyond compare. I never get tired of listening to them. Moonlight Sonata is arguably the greatest piece of music ever written.
Excellent topic! :thumbsup: