This is a great question, Romy! Who knows, really, since music has gone more and more "pop" for the last 50 years and the "average American" is probably stuck with the notion that this stuff is the music of the 20th century. Well, never mind that; but what do we have in the way of music from 490 years ago? Not much, really.
But I have been thinking about a similar question for many years now, namely, what/who is the most important Art/Artist of the 20th century, using the today's "informed" standards and "projecting them into the future"? I don't know about 490 years into the future, but maybe in 100 years or so...
At this point in my life, I would have to say that Marcel Duchamp was the 20th C visual (conceptual) artist, Joyce was probably the writer, and American Jazz (broadly speaking) has been the music to "define" the 20th Century. If held down and forced to choose, I would choose Miles Davis as the musical artist of the 20th C; not Schoenberg, Philip Glass or John Cage, but Miles Dewey Davis. And so I suppose it's tres droll that I think that Sketches of Spain is probably Miles' greatest work.
It is very interesting how time works to "temper" our views. Who can listen to Mozart without hearing Handel, and so on, ad infinitum. All artists have their predecessors; it's just a matter of knowing whom and how.
True, I do not rate "Sketches of Spain" as highly as Brahms' 4th (or Beethoven's 9th...), etc; but Art is always the Voice of its own time; and ultimately I think it will be so appraised.