Female: Julie Fowlis, Anne Hills and Debra Cowan have been mentioned, and I heartily agree. I'd like to add Padraigin Ni Uallachain, Anne Dodson, Sofia Karlsson and Sissel Kyrkjeboe.

Male: Gordon Bok is No.1, but Liam Clancy, Theo Bikel and Ed Trickett are also really good. Waylon Jennings and Don McLean both sang a few folk songs early on in their careers, and did it beautifully.