I generally reject the idea that anyone was the best at anything when it comes to art or music. For me, art is about communication and transcendence. If someone can draw you into their world and show you something that you like that's really all that matters. I don't have the interest or the talent to say anyone is the best. There are however some artists who do tempt me to say that they are the best and Otis Redding is one of them. If someone wrote that "Otis Redding is the best soul singer that ever existed" could anyone disagree with that? I mean if you're not Al Green, David Ruffin, Wilson Pickett or Marvin Gaye, could you? And if you mention D'Angelo or Mayer Hawthorne I'm going to throw something heavy at your head. Seriously. You should duck.
Unfortunately Redding's life was tragically cut short in a 1967 airplane crash that also killed most of the Bar-Kays. So we never got to hear everything that he was capable of doing. He was really just getting started. I don't listen to a lot of modern R&B. One reason I don't, besides a generalized distaste for pseudo-disco, excessive melisma, synths and drum machines, is that I just haven't heard many modern male singers that have the kind of power and control that was exemplified by classic soul singers and most particularly Otis Redding. He was nicknamed "The Big O". (He stood 6-4 though judging by how some women carried on at his concerts there may have been other reasons for that sobriquet)