It's difficult to define who is and isn't a "good" singer as so much depends on the style. A good classical singer might sound naff singing folk music whilst a good folkie probably wouldn't sound entirely convincing singing grand opera.

For me, one of the nicest parts about the folk world is the discovery of unknown artists who turn out to be brilliant. I heard George Papavgeris the other day- OK, he's not unknown but I'd never heard him before. For me he ranks up with the Dylans and McTells of the world in his songwriting ability and, though he doesn't have a "good" voice in the conventional sense, his voice exactly fits what he does. Which is what it's all about IMHO.