Cool thread, lots I would've listed as well have already been done. I haven't seen;

Brian Wilson
Nick Drake
Van Morrison
Warren Zevon
Joni Mitchell
Willie Nelson
Johnny Cash

Or how about say; George Gershwin, Duke Ellington, Rodgers and Hammerstein? Guys like that practically invented popular music.
Thelonius Monk, Bill evans, Miles Davis for straightahead jazz composers, etc. Will post more I'm sure when they come to me.