Lucinda Williams
Joni Mitchel
(now it gets tough!)
Gonna go with Barb Jungr mainly for her Dylan covers.
Honorable mention:
Patricia Barber
Eva Cassidy
Emmylou
Iris Dement
Gillian Welch
Betty Levette
Flora Purim
Bonnie Raitt
Margo Price
Chrissie Hynde
Margo Timmins
Janis and Grace
Lydia Pence
….and many more