its so hard to tell what songs are 50/50 or 80/20 apart from A day in the life, but i found this list to that helps if it is correct.
so going on that list id say
worst: I wanna be youre man
best: She loves you
That list isn't accurate in a few places, and it's dangerous because it doesn't let the "casual" fan know that John and Paul collaborated one way or another on most songs. For example, Paul wrote an entire verse in "Lucy In The Sky", and John contributed to a lot of lyrics to "Penny Lane". "With a Little Help" should be in the Paul list because he originated it, and he and John finished the lyrics together. John wrote the intro to 'Ob-La-Di", Paul contributed the bass line and overall feel to "The Ballad Of John and Yoko" etc.
My point is, lists like this make it appear that there was less collaboration than there really was, and that detracts from the importance of the songwriting partnership.