I see 'voice' in the sense of this thread as being the over-all result of understanding/loving the material being presented and then presenting it well.

I think Stan Hugill is another 'best' voice in folk. Technically, his voice when he does shanties/chanteys is horrible BUT, I listen to his work and because of him have come to understand that the days of sail didn't require a Pavarotti to synchronize the heaves and hos. Thus, he's a 'best' singer, imo.