This was just brought up in another forum in reference to Paul McCartney as being an overlooked talent on the bass. I'm going with Flea and Bootsy. Bootsy was only 18 when JB brought him onboard. Whatever anybody thinks of James, he did know players.
In a non-rock category - 1/2 of the Riddim Twins - Robbie Shakespeare is certainly one of the most recorded as it's estimated he's been on over 200,000. tracks.