I'd have to say Paul. I've never been much of a John Lennon fan outside of his days with the Beatles. Like many out there, I can't stand the sight of Yoko Ono, and the wedge that she drove between these four great musicians. I think John and Yoko were into too many weird things post Beatles breakup, while Paul was writing great stuff with Wings.
Many will say that Paul is too sappy, and the music is a bit contrived. I say B.S to that! I think his song writing was an amalgamation of what was going on in the world. John's writing had too much tunnel vision connected to it. His music seemed to be surrounded with what was going in Vietnam, and involved everything that was cynical regarding the U.S/our position in the world, etc... Paul was definitely the eternal optimist, and unfortunately got crap for that. I say, whatever!
Ringo and George did their thing, and they did pretty good....though Ringo never saw much success beyond the 70's, while George had a little bit of a resurgence in the late 80's. Though I much prefer the music George was making in the 70's.