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.
I think a woman's a stronger opponent because it takes "It's time for a female President" off the table. There's no other milestone that's as significant, at least in terms of what will energize voters. A woman who would also be the first Hispanic President is the only thing that would be bigger than just the first female President. And it would complicate the campaign for the Democrats. They can't rely on accusing the Republican campaign of sexism, and there's the possibility that someone will say something stupid to offend Hispanic voters.
Another option would be someone who is accomplished, widely known and admired by the public. That type could run an outsider campaign. Think Bill Gates without the atheism.
