Ringo seems like a really nice guy who was scrambling to keep up with his peers, and never really succeeded. As most people have noted, Watts was/is noticeably more rock solid. That Watts is far more serious about the art of drumming is reflected in the fact that he's still very active, and stretching out into other arenas. (I'm under the impression, btw, though I could be wrong, that Paul McCartney did a certain amount of the drumming on Abbey Road.)
I don't think either would qualify as "great" drummers, frankly, but Watts would be closest. Whoever mentioned John Bonham...now he left his mark. Another hugely under-rated drummer was Clive Bunker, who was with Jethro Tull through Aqualung (I think). One of the most creative AND rock solid drummers in rock 'n roll.