that may well be true for the sixties, but rrushe didn't specify an era. you couldn't dismiss (for example) rodgers & hammerstein, cole porter, etc, from earlier in the century.
20th century........The Beatles with most album sales. Yesterday by the beatles is the most recordered cover song with over 2500 versions which gives the song writer Paul McCartney plenty of dosh to give Heather
Fats Waller, Rogers & Hammerstein, Hoagy Carmichael, Irving Berlin, Gilbert & Sullivan, Bob Dylan, The Beatles, Brian Wilson, Lieber & Stoller, Chinn & Chapman, George & Ira Gershwin, Isley - Jasper - Isley, The Gibb Brothers, Amy Grant, Curtis Mayfield, Lorenz & Hart, Goffin & King, Jagger & Richards and many, many more all could claim that from different times. Would anyone like to add more?