I agree with Art Thieme that Peggy Seeger is a superb banjo player and accompanist. A few years ago I heard her accompany herself on a song called "The Rambling Gambler" and it was absolutely knockout! Unfortunately, the only recording I can find of her singing this song is an early one on which she uses the guitar. The last time I saw Peggy I requested this song but she didn't have the long-necked banjo with her.
Two more women singers who use a banjo are Shirley Collins and Alison McMorland. They each have very different styles (different again from Peggy's absolute mastery of the instrument) but I find them both to be very effective self-accompanists.
This is a very subjective view, from a non-instrumentalist, but the accompaniment of folk songs on the banjo summons up for me an older, deeper tradition and puts me in mind of those amazing singers from the Balkans and the Middle East who accompany themselves on a variety of stringed instruments.