As awesome as John Lennon is, it'd have to be without a doubt Johann Sebastian Bach.
I mean fuck guys, you're all picking musicians who were around during the recording era, we can still hear the if we want to. And quite frankly, pop musicians who just are not as impressive as those old classical motherfuckers.
Take a listen to the genius of Bach's music, music that defined the tonal system as we know it. And now realize that in Bach's time, he was remembered more for his skill as a PERFORMER than as a composer. Think about that for a second. I mean, they say the man could IMPROVISE FUGUES, which is just disgustingly insanely awesome.
Ya, definately Bach.