I'd put McCartney at the top of the heap for both Beatles and post-Beatles. I'll also say that he's on a pretty great hot streak right now. Chaos & Creation..., Memory Almost Full & the latest Fireman album are all great, non-coasting artistic successes, given the fact that there has always been a cloying "love me" quality to his music.
If you read Ian McDonald's "Revolution in the Head," which you should really do if you like the Beatles, he writes that Lennon's melodic sense kind of abandoned him post-Revolver and that he was basically saved by innovative production in the latter albums. I would say that he lack of melodic invention kind of dooms most of his solo work for me as well. Lennon was more about passion than craft.
Given all of that, no Beatle ever made a better solo album than All Things Must Pass. That's just a fact, and Living in the Material World was almost as good. Unfortunately, George's albums kind of fell off a steep cliff after those two, although I really like Cloud Nine and the Travelling Wilbury's first album.