Best Classical music piece of all time

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There's this one Chopin piano concerto that has also been done as a piano/violin concerto. (I'm pretty sure it was originally written for solo piano). I don't even know the opus or anything but it is my favorite piece of classical music. There have certainly been some more astonishing pieces and there are many better but it's my favorite.

It's pretty well-known, I think it's in C or C#.

NT