Rubin is talking about McCartney being a unique songwriting talent and a great singer. It's a bit of a stretch to include the title, best bass player!
IMO the number one, the most significant bass player of all time, is the guy who changed the way that bass lines were created and changed bass playing for everyone.
The guy who didn't just open the door for the rest of us, but knocked it clean off it's hinges. The guy whose bass lines were unique and ranged from the simplest, Higher and Higher and My Girl to the most complicated, For Once In My Life and Darling Dear . . . James Jamerson.