Denverdoc named a couple of my all-time favorites--Joan Baez and Kate Wolf--but, like he said, folk artists have been neglected here. Some Brit ladies I really like are:
Sandy Denny (Fairport, Fotheringay, solo)
Jacqui McShee (Pentangle)
Maddy Prior (Steeleye Span)
And I don't believe anyone's mentioned Yma Sumac--"Inca Princess" of the five-octave voice. Most of her material was, unfortunately, wretched, but one song you should absolutely hear is "Chuncho-the Forest Creatures." Here, accompanied only by a classical guitar, light percussion, and occasional flute, she imitates various animals of the rain forest, from the deep baritone growl of the panther to the chattering of monkeys to the twittering of birds. And it's a decently composed piece of strangeness, full of interesting dynamic shifts, and drenched in mid-'50s analog reverb. Mono recording, but with depth.
