I find it interesting how people go off at a tangent and ignore the original question, answering instead a more general question in their minds, like "who do you like?". Paul Robeson was the greatest bass/baritone that ever lived, sure - but not a folk singer per se. Neither was Harry Belafonte. And a number of songwriters were mentioned for the strength of their songwriting rather than their voice qualitis, which is what the question was about.

In fact one is more likely to find good voices among non-songwriters; after all, performing alone is their strength.

I forgot earlier to mention one voice who is said to be among the closest to delivering traditional English song in the traditional unadorned way (so I am told, I wouldn't know as I have scant knowledge of traditional methods of delivery). I just love the quality of his voice and his unassuming, unaffected manner: Geoff Wesley.