Yesterday may be the most covered song? but Mc Cartney or Lennon did not have the songwriting skills of myriad American writers like Porter, Berlin, Lerner, not to mention the great contemporary writers like Simon, Goodman, Chapin/Carpenter, Prime, Mitchell and dozens more.
The Beatles stuff though popular is mundane and lacking in depth, even "Imagine" is a simple anti war dirge when stripped down.
Ray Davis is undoubtedly a superior songwriter, even if he has not produced in the quantity of M and L and there are plenty of excellent writers in the contemporary idiom here in the UK. Unfortunately their talents are seldom recognised Pete Bond being a good example.
Gerry Rafferty could have knocked Mr McCartney into top hat.