At its core, attractiveness has always been a quality that stems from one's likelihood to bear successful offspring. With the advent of society, that's fluctuated over the centuries, from being overweight as a reflection of wealth, to being thin as a reflection of self discipline in the face of horrible diet options.
But ultimately, despite how far removed from the food chain the human race has become, strength and fitness will always be the fundamental arbiters of one's ability to survive; and consequently, for those whose minds are in the right place, the primary determinant of who is or is not "attractive".