I agree that of the four, Harrison was the most talented. His post-Beatles work proves this quite handily, IMO.
As far as Lennon v. McCartney. Harder to say. Apples and oranges. McCartney wrote/writes more pop-oriented stuff, Lennon's work a bit more cerebral. Singly, they both produced some really good music. together, they wrote some of the best rock/pop music ever.