Sam Cooke hasn't been mentioned yet??
And definitely Otis Redding, Marvin Gaye, Aretha Franklin, Nina Simone, Bobby Womack, Jackie Wilson, Al Green, Luther Vandross, Smokey Robinson, Donny Hathaway (sampled many times in classics), Luther Ingram (sampled by Madlib and Dilla), James Brown (what would hiphop be without him?), Ray Charles, Stevie Wonder and Otis protege Arthur Conley (extremely underrated, his voice is simply amazing check out Sweet Soul Music)
I see Curtis Mayfield got mentioned. Believe me the best Curtis, is Curtis with The Impressions the legendary soul group. Most people seem to like Curtis as a solo artist more but his vocal range was simply brilliant when he was with The Impressions, it was actually pretty close to Sam's. Listen to Gypsy Woman by The Impressions and you'll get what I mean but fuck that corny Watch The Throne shit, they disrespected a legend like Otis putting cars and shit like that in their clip and using his voice.
Also Marvin Gaye's best album is Here, My Dear, it's better than What's Going On