I'll make the first one the obvious one.

She writes her own songs and music (except the occaisional cover version)and 16 out of her 17 #1 hits are hers and hers alone.
Mariah Carey of course.

*sigh*
Also the other greats:
Carole King
Joni Mitchell
Joan Baez
Maybe Stevie Nicks and Christine McVie
In this decade Alicia Kys
I can't think!!
I have many other female "singers" I like but they're not songwriters per se.