Greatest Songwriter of all time

I was reading the Dylan post and it got me thinking. There are many great songwriters that come to mind, my expertise/knowledge is primarily with rock music. If you could only pick three, what would they be? My vote for best lyrics goes to:

Jim Morrison
Bruce Springsteen
Elvis Costello

Best music goes to:

Mark Knopfler
Stevie Ray Vaughn
Frank Zappa

There's many great ones, but ones that belong in the "best" conversation for songwriters (which implies popular music with lyrics)

Joni Mitchell (no one else in her league that can compete solo in musicianship, lyrics and music),  Bob Dylan (not one of my faves but belongs in the conversation), Lennon/McCartney, Neil Young (not the greatest, but tied for favorite) George Gershwin.