A catch for Penn, but is she really "the most important" 20th-century female philosopher?  De Beauvoir?  Murdoch?  Ruth Barcan Marcus?  Philippa Foot?  Marjorie Grene?  Susan Stebbing?   Some of these folks are less read now than Anscombe, to be sure.   And Anscombe had greater breadth than some of these figures.  (I've commissioned volumes on Anscombe, and on DeBeauvoir, for the Routledge Philosophers series, but none of the others at this point.)

UPDATE:  A couple of readers rightly mention Hannah Arendt (about whom I have commissioned a volume in the Routledge Philosophers series), although Arendt is less read than Anscombe in the confines of what remains of "analytic" philosophy.