Well, most people would rattle off Paul - John - George - Ringo, in that order. But I think John deserves more credit than is usually given (about his musical talent, I mean). First of all, I think he has a great voice -- not conventional perhaps, but certainly unique. Secondly, I don't think he was trying as hard as Paul to make good music. I remember reading somewhere that Paul would have a bunch of songs ready and *that* would motivate John to come up with some stuff of his own.
About George -- although he is perhaps my favorite Beatle, I think he was a decent guitarist, decent song-writer and has a decent singing voice, but that's about it -- nothing really awesome.
Ringo, in my opinion, could have been replaced with any average drummer and it would have made no difference to the overall music. Although, to his credit, he did keep in time well - like a human metronome (but then that's the least expected from a good drummer).
Bottom-line: With the Beatles (no pun intended) the sum of all parts was far greater than the individuals.