Best female manly voice?

I am a high school teacher, and at lunch a group of students eat lunch in my room. I usually try to play different kinds of music just to expose them to something other that MTV and the Top 10. After listening to Cassandra Wilson for a while one day, one student remarked, "For a woman, she has the best man's voice I've heard." And he wasn't making fun---he went out and bought a CD.