These are my views on each Beatle:
John: A lyrical genius who was extremely witty, John wrote masterpieces inside and outside of the Beatles, though his songs had more emotional depth to them. Listen to Plastic Ono Band and you'll understand.
Paul: The melody master who wrote good lyrics with many double entendres. Once free from The Beatles, his lyrics had more meaning but he still wrote some raunchy songs (Monkberry Moon Delight, Eat At Home.)
George: Though he wrote some beautiful songs, both John and Paul's egos were too big to let George have any more than 2 or 3 songs on an album (aside from The White Album). The songs that were rejected by John and Paul spawned an enormously successful triple album. George was an underrated genius who never got his time to shine (though i doubt he'd have liked the spotlight).
Ringo: A severely underrated drummer whose influence spawned many other great drummers. Not the strongest songwriter though...