I suck at picking single-best anythings:

Second Andy Stewart for voice, although I'm not that familiar with the range of the material.

Slaid Cleaves for pretty songwriting.

Dylan, Woody Guthrie, and (sorry, I have to say it) Steve Earle for content.

Leadbelly, Mississippi John Hurt, and Norman Kennedy because I can't believe they wouldn't qualify.

The whole of the original Red Clay Ramblers for Best Use of Nasal Singing.