I wouldn't pick either of them. If I had to pick just one songwriter/performer, it would be somebody else. Neither Lennon's nor McCartney's solo work is good enough to crack my top list of songwriters and performers. They both had some very good stuff, and Band on the Run is right up there, but if they hadn't been the Beatles first, neither of their solo catalogs is consistently strong enough to warrant consideration. McCartney is too light and far too self-conscious, and Lennon is way too cynical and preachy. Together, they set the standard by which all others are measured, but individually they are both seriously flawed. There is no greater example in all of life of the whole being greater than the sum of the parts.