@mrskeptic: You misunderstand my intent in the use of the term "greatest" (but @tony1954 doesn’t). It was deliberately chosen not to apply that adjective as a simple-minded statement of fact, but rather to make a polemic assertion. My statement assumes all readers are aware of the fact that in art there is no greatest, except for the music of J.S. Bach, of course.
Still, in the world of Pop music songwriting, "God Only Knows" stands alone. Says who (or is it whom, rpeluso? 😉)? Me. And Paul McCartney (a pretty good songwriter himself). And a few excellent songwriters I personally know (or knew. A few are dead).
But I have plenty of others. "The Weight" by J.R. Robertson, "A Whiter Shade Of Pale" by Gary Brooker & Keith Reid, "Nowhere Man" by John Lennon (which contains my favorite bit of George Harrison’s guitar playing in the song’s short/perfect solo), "Waterloo Sunset" by Ray Davies, "Waterloo" by Benny Andersson and Bjorn Ulvaeus, "Shake Some Action" by Cyril Jordan and Chris Wilson, "Fresh As A Daisy" by Emitt Rhodes, "Why Don’t We Talk About It" by Rodney Crowell (found on his perfect album The Houston Kid, which I rate higher than Pet Sounds), "No Time To Cry" by Iris DeMent (about which and whom and I learned of in an interview with Merle Haggard), "Love Hurts" by Boudleaux Bryant (the original by The Everly Brothers please, not the inferior remake by Gram Parsons and Emmylou Harris), "Hallelujah" by Leonard Cohen, and "Like A Rolling Stone". There are dozens and dozens more I could name. All great, unique, and incomparable with each other.
For me, it all starts with the song. That’s why I don’t love than many Rock bands: most don’t have a great songwriter amongst it’s members. Being a musician, even a very, very good one, has NOTHING to do with writing songs. Prior to Brian Wilson and Lennon & McCartney, Pop music was most commonly made by separate teams: professional songwriters, studio musicians, the featured singer(s), and the record producer. After The Beatles, all Rock bands were expected to write their own material. That has resulted in a lot of mediocre music being made. IMO, of course..