As has been said, there is no "best". Too many great ones to choose. Sinatra cited Billie Holiday as an influence and favorite. Karen Carpenter had an angelic voice, as does Alison Krauss. 'Retha was an astonishing singer, just killer. Gladys Knight, too. Susan Tedeschi (nice guitar player too). Too many to name, too many to choose just one.
One I discovered recently is Beverly Kenney, who's the singer of "It's a Most Unusual Day" in the Lincoln Town Car ads that were prevalent not long ago. Turns out she was a 50's jazz singer who died tragically young, but had a nice career going, performing at Carnegie Hall with a number of jazz luminaries and putting out several top notch albums. I thought I knew all those singers, but I'd missed her somehow. A sweet and supple voice.