Nat King Cole and the underated Marilyn Monroe would get my votes. One had the most perfect smoothest style ever whilst other dispensed entirely with artifice.
I'm also constantly surprised at what a great voice Elvis Presley had, and Doris Day could make almost any song sound good. Matt Monroe and Michael Holiday all most overdo the smoothness. Judy Garland had almost everything, except, understandably, a sense of ease.
Although the song always comes first there is something primal in the way a human voice sounds.