I'd say Ani DiFranco but i don't consider her in the "Jam Band" genre. I consider her folk. However i love the little folk singer and she's one of my top 3 artists. If the question was Best Female bands/performer then Ani would win hands down for me.
I'd have to go with Grace Potter (GPN). I only saw them once but she/they blew me away.
If the question was Best Female in a Jamband ... then Allie Kral of Cornmeal.