I'm really a Stones guy, but I love the Beatles. Putting songwriting aside and just going with playing an instrument, it seems to me that Paul really put in the time practicing as the Beatles progressed. Ringo is consistently great and George put out really good solos and guitar sounds (his lead work on the last few records is crazy good), but Paul sounds like he tried to become a better bassist as the years went by. I just listened to Sgt. Pepper's with the headphones and was amazed with Paul's playing. His work on later albums is fantastic, but it really hit me as I listened today just how much his bass drove Pepper's. Songwriting is so important and Lennon wrote some amazing songs, but I want to give a shout out to McCartney. The Beatles later work would not have made it without his playing.