Ray Charles was the originator, the genius and the father of it all. That man could pretty much do no wrong. Had more sheer musical talent in his little finger than my entire band combined. Not only that he was a cool motherfucker. Just finished his memoir and LOVED it. There is a live version of You Don't Know Me from 1964 that almost brings me to tears every time I listen to it. It's on the Rhino Handmade In Concert. Pricey but WELL worth it.
http://www.rhinohandmade.com/browse/ProductLink.lasso?Number=7826It's a sampling from various live albums from his Atlantic and RCA years. Most of it is out of print except for this volume. Every performance on here is amazing.
BUT
Marvin Gaye was...dude he was Marvin Gaye. Let's Get It On may be the best album to come out of the 70s. It's one of the few albums that I consider perfect (Revolver, Bringing It All Back Home, The Joshua Tree and Definitely Maybe round out the list for me.) Here, My Dear may be one of the most moving albums I've heard.
BUT
Sam Cooke probably had the best voice. I could listen to his gospel work with the Soul Stirrers all day long. Its so full of power that it gives me chills just thinking about it. The show from Miami's Harlem Square Club is mindblowingly good. Probably the greatest R&B live album of all time (even better than the first James Brown Live At the Apollo)
I grew up listening to Smokey, The Temptation, Jackson 5 etc. so R&B makes up a huge part of my life.
If I had to choose just one though it would probably be Brother Ray. He was a HUGE influence on the other two as well as Aretha.