As some have already stated: any woman that would have me. And that's the damned truth.
Moral of the story: be grateful you have a woman that will put up with you.
Now, to be fair (and answer the question): I would say, automatic arousal for me is most often provoked by bottom-heavy, pear-shaped ladies. But there is also a myriad of factors at play.
For one thing, what is actually being flaunted? Presentation counts for a lot. Also, one's own sense of security.
I mean, if Milena Velba were wearing a hazmat suit, you might not notice her.
Anyhow, I have been fortunate enough to have crossed paths with some pretty sexy women in my time (not that I had any chance with them), and who were in fact neither top-heavy nor bottom-heavy. Some women have beautiful faces, beautiful hair, stature, hands, and/or voices even. And some--if you're really lucky--have an aura that just fills you with contentment, breathing the same air. As if you were in the presence of the maker.
Frankly, even a nun can be sexy--i.e., someone going out of her way not to be! Because sexiness and true confidence are in the soul of the individual. And it's a lot of different qualities.