Since I've just noted her absence here, in my Mudcat thread about a Song of the Day series at my own site, I'd say Eliza Carthy on her day should be in there with a shout. Linda Thompson, too, at her height.

And while I know it's deeply unfashionable to say so, I still love Joan Baez's voice.

But we are spoilt for choice. Looking back over this thread, I can say without even checking that Mary Black, Cara Dillon, June Tabor, Dolores Keane, Chris While and, of course, Sandy Denny have all appeared in my selections, along with a few of the others mentioned.