First name that came to mind was Helen Schneyer. Then had to give equal time to Jean Redpath, Lisa Neustadt, Ray Fisher, and Lisa Null. All of these women have powerful, individualistic voices and styles. But they are less concerned with their own singing style than with conveying the meaning and feeling of the song. Most of them have huge repertoires of traditional music.

For men, my first thought was Dwayne Thorpe. Dwayne is a college professor who sings "Zebra Dun" like he really means it. Wide range of material, fine guitar work, a very expressive and versatile voice, great on both old-timey and blues. I only know one album that he's done, darn it. Lou Killen is superb, Allan Damron is wonderful, can we name our top twenty? Or top forty?