Susanna Martinez comes close. She's a popular swing state Governor from a region where Hillary is weak, and she would represent a bigger milestone. She'd be slightly stronger if she had kids, and took stands on civil liberties similar to Rand Paul.
I'm not sure a woman is actually the best answer to Hillary. Hillary's had so much time to build her brand among female voters, and there's still the bad memories of Sarah Palin that might make some women voters leery of a Republican woman candidate.
I think the best way to counter Hillary is with a male candidate, likely a minority, who can energize male voters while not giving the Dems any way to play the war on women card that will help Hillary. Essentially, you can't beat Hillary at her own game. You have to find a way around it.
