Probably Bobby Keys. Primarily for his work on Let it Bleed, Sticky Fingers, and Exile on Main St..
Maceo Parker, if R&B/soul/funk counts. And a handful who played on thousands of Motown and Stax records.
Michael Brecker recorded on a ton of pop records as well. Most recognizably maybe on ‘Still Crazy After All These Years’.
My favourite guest appearance is Ornette Coleman on Joe Henry’s ‘Richard Pryor Addresses a Tearful Nation’. Much like the song title, it is both absurd and sublime. Coleman did very little work as a sideman, and the only other rock/pop musicians that he recorded for than I know of are John Lennon and Lou Reed. It’s insane that Henry talked him into it, but I’m glad that he did.