I'd have to start with Bonnie Raitt ... her voice and her guitar work just knock me out!
Two at the top of the list have to be Joan Baez and Joni Mitchell. Joan for her "stay at home" guitar stylings and Joni for her imaginative tunings and composition, both for their beautiful voices!
Maybe - Rock - Joan Jett, Folk/country Emmy Lour Harris, somewhere in the middle Nancy Wilson
All the women stated so far are on top my list. One of my favorites is Patty Larkin. When I first started to learn the guitar I saw her in concert, man, what an inspiration.
Susan Tedeschi, Shawn Colvin, Vici Martinez, Alison Kraus, Brandi Carlile, Gillian Welch, Jewel
Sue Foley - paisley pink guitar and all...and as Tom's said, Susan Tedeschi...both amongst the very best of blues guitarists...
I slipped onto the stage, way back when, and asked Nancy Wilson (Heart) for a date. She refused, so my fav defaults to Bonnie Raitt as a guitarist, but not singer.
Did Bonnie say yes? Singer? I loved Gracie Slick.
After Nancy, I lost my confidence....
Bonnie Raitt !!!
Gilllian Welch and Joan Baez. Ms Welch is one heck of a song writer/singer/guitarist.