Listing only those I've seen in person and in no specific order:
Jean Ritchie. Jean Redpath, Priscilla Herdman, Anne Hills, Cindy Mangsen, Joan Baez, Judy Collins, Maddy Prior, Susan Reed, Peggy Seeger, Connie Dover, Buffy Ste.-Marie, Jane Wilcox Hively, Custer LaRue, Ann Mayo Muir, Bonnie Rideout.
Among those heard but not seen: June Tabor, Joni Mitchell, Rosalie Sorrels, Ann Dodson, Judy Small, Cynthia Gooding, Cathy Barton.
Overall favorite? probably Jean Ritchie, but changes depending on whom I'm listening to.