I also have this board to thank for the discovery of early solo McCartney, but as far as I'm concerned Lennon is responsible for the overwhelming majority of great Beatles songs. Almost all of my favorites are John's:
Tomorrow Never Knows
I Am the Walrus
Strawberry Fields Forever
Don't Let Me Down
Come Together
Across the Universe
Dear Prudence
A Day in the Life (partly Paul's, but mostly John)
She Said She Said
Paul wrote some great Beatles songs too, but not nearly as many as John. And he wrote Ob-La-Di, Ob-La-Da, which is a pretty large check in the negative column.
Within the Beatles:
John
Paul
George
Ringo
Solo:
Like I said, I'm discovering (and appreciating) early McCartney. I agree with yesno that the first 2 Lennon solo albums are great (actually, I only like about half of Imagine, but the debut is a classic). George's All Things Must Pass is unquestionably the best album by a solo Beatle. I don't really know much beyond a song or two about the albums he released after All Things Must Pass, but he gets #1 anyway. Ringo is last by default because I know only a couple of songs.
George
Paul
John
Ringo
Overall:
I'm torn. In fact, I've changed my mind between voting in the poll and typing this post. Using a totally non-scientific ranking system, I'd probably rank Paul at overall #1. He took longer to nosedive into light-rock garbage than John (and his oldies album totally thrashes John's) and he's been responsible for more great moments than George. Overall, he's been the most consistent of the four.
Paul
John
George
Ringo