I'll have to ditto Maria Muldaur. When I first saw her perform she drove me nuts (it was with the Kweskin Jug Band). I've seen her a couple of times in the past few years. She just gets sexier (better) and sexier (better) all the time - and she's still singing "Don't you feel my leg".

Maura O'Connell is right up there, too.

On the male side, I'd have to say the bass singer for the Fairfield Four (gospel quartet). He's another one who could just read the phone book and send electrodes through the system.