I'm surprised no one has yet mentioned Cozzolani (Chiara Margarita C,
c. 1602--1677), whose music comes closer to greatness than any of the
merely very good female composers other listers have mentioned -- her
music can pretty well rub shoulders with Monteverdi's, and that's good
enough in my book.
If you're interested, try the _Vespro della Beata Vergine_ and _Messa
Paschale_, both performed by Magnificat directed by Warren Stewart on
Musica Omnia -- respectively MO0103 and MO0209.
Toccata Press/Toccata Classics