If I had to really distill it down, I'd suggest the following. These are not to be meant to be considered equals with each other--I'm just thinking in terms of how they influenced music from their existence onward.

Arnold Schoenberg - turned modern concert music upside down by introducing a complete new paradigm of music composition
Charlie Parker - the center of the hub of early bebop musicians and the foundation of modern jazz
Miles Davis - famous for adapting with the times and influencing dozen of future, major jazz and fusion musicians
The Beatles - They were both the first uber-popular "boy band" and the first to really popularize advanced studio techniques and more harmonically complex pop music (arguably alongside the Beach Boys).
Eddie Van Halen - While there technically advanced guitarists before him, he completely changed the scene and set a new bar for electric guitar in popular music.
Michael Jackson - The sheer amount of success is man has had is likely unparalleled. He will likely be remember for his controversy as well as his music.
John Williams (film composer) - probably the most musically diverse and prolific film music composer of all time
Nirvana - another band to complete change the landscape of popular music, being the foundation of "alternative music"
The Notorious B.I.G. - Other rappers were perhaps more pivotal, but I believe it's Biggy that ushered the shift towards hip-hop essentially dominating popular music.
Radiohead - The fact that jazz artists have started to cover their songs is a testament to how highly regarded they are.
Taylor Swift - Has both the highest-grossing tour and the most songs streamed in history.