Being the first doesn't mean she's going to be popular. It's probable that half the country will loathe her and she may just last one term.
If there's any woman who's the most likely, its Nikki Haley. She was the Governor of a very rough-and-tumble state in the Deep South, she won despite not being white, and she was re-elected. That really is all the qualification she needs, but now she's in an international role and she can very easily run as someone who has foreign policy experience.
I have a hunch that she will be a Governor, not a Senator.
Haley almost lost to a Democrat in a Republican wave year. I can't see how she could be President.