Per question posed from Die Domask who shared this article

I have no idea how to do 'best.' But I can make a basic list of the composers of the last millenium without which music as we know it today would not be possible, if I got it down to 10 it would probably be:

Perotin
Machaut
Josquin
Monteverdi
Bach
Haydn
Beethoven
Wagner
Ellington
Dylan

Possible Honorable Mentions:

Ockeghem, Dufay, Mozart, Stravinsky, Sondheim

Maybe tomorrow I can do a 20th Century List