I would like to add a couple of soul greats; firstly, Mr. Soul, the incomparable James Brown. He sure liked beating his wife, but he could suck every last drop of emotion from a melody like a 200-watt vacuum cleaner sucks dirt from a carpet; secondly, my fave singer of all time, Sam Cooke. He could do it sweet and clean, or he could get down and dirty, and he wrote nearly all of his stuff -- some of which are the most exquisitely-constructed yet deceptively simple melodies in the history of popular music.
And I also love sinatra, who had many of the same qualities as sam. Plus Hank Williams, but that's more because of his wonderful lyrics and mournful soul than the timbre or clarity of his voice. for that particular distinction, I would award the greatest natural singing talent as the late Johnny Cash. His voice soared and seared, infusing the Cash flavour to everything he set his vocals to. So many of his covers reached greater peaks than the originals that I think that if I could only listen to one more singer for the rest of my life, it would be him.
I'm just thankful for all the great singers mentioned on this thread because they have all brought such great joy and light to the world, and will continue to do so, even though many of them have passed on.
P.S. R. Kelly is not included in this group. Sorry -- peeing on young girls is a bridge too far as far as I am concerned.