The Arc of the Moral Universe is Long, but It Bends Toward Xmastime

Moi Say Moi:

...while John Lennon wrote a million great songs in The Beatles, anytime you hear anything "interesting" in one of his songs, the odds of it being because of Paul is about 90%. 

 

Aaaaaaaaaaaaaand since the Universe is finally catching up to me, here's an excerpt from I an Leslie's newly released John & Paul: A Love Story in Songs:

McCartney's bass powers the song from beneath. In the middle section he walks down the scale as the vocal line moves up, an early example of his ear for the contrapuntal. "Please Please Me" is a model for how McCartney would impose himself on his partner's songs. Through his talents for arrangement, singing, and playing, he helped to turn a somewhat maudlin, aggrieved ballad into a cruise missile carrying a payload of joy.

Another great example I pointed out a coupla years ago is Ticket to Ride:

The song is primarily John’s in the writing, but the arrangement is McCartney’s work — in this middle period of the group’s career most of their truly astounding work comes when Lennon brings in the song but McCartney experiments with the arrangement and production. Over and over again we see McCartney taking control of a Lennon song in the studio and bringing out aspects of it that its composer either had not considered or had not had the musical vocabulary or patience to realise on his own.

It goes on to explain how it was Paul who came up with the song's iconic drumming and guitar line. In other words, pretty much what makes the song the song.