For me it would be Ike Turner.
OK, he was an arrogant, cokehead, nasty piece of work, but he started it all in 1951 when he played piano on "Rocket 88" by Jackie Brenston (classed by some as the first RnR record). During the 50s, he had numerous records on Federal / King with his Kings of Rhythm, and then his work with Tina Turner from 1960 onwards.
He remained a soul and blues musician right up until his death in 2007. 56 years of work (minus a few years of incarceration) must surely put him up there as one of the highest-ranking soul brothers.
Mind you, Joe Turner would also be a strong contender - his career starts in the mid '30s as a singer accompanied by Pete Johnson, Albert Ammons and Meade Lux Lewis. He died in 1985, so probably didn't notch up quite as many years as Ike as a recording artist.