I believe the best is a toss up between:
Joan Jett and Lita Ford, absolutely amazing players and musicians. Love their music!
My favorites certainly have to be:
Joan Jett (The Runnaways, Joan Jett and the Blackhearts)
Lita Ford (The Runnaways, Solo)
Nancy Wilson (Heart)
Donita Sparks (L7)
Katherine Thomas (The Great Kat)
Bonnie Raitt
Flavia Canel (Drain Sth)