The most attractive thing on earth is obviously going to be somehow related to humans of the female persuasion, but you can’t pin it down to any one physical trait, because besides being very shallow, it pretty much varies with peoples personal preference.
I think the number one specific thing that most guys will consistently find attractive in a girl is basically a certain attitude. Strength in vulnerability. This has nothing to do with being emotional, in fact, most guys are seriously turned off by too much emotions. On the other end of that scale is when strength is taken too far it becomes hard, which is also a big turn off for most guys.
(There is always the exception, some guys are attracted to emotional girls, and some find hardness sexy; but from what I’ve seen, they’re both the sort of guys that girls should avoid.)
The perfect mix is when a girl can be genuinely vulnerable with how she is feeling, and yet despite that be strong. Genuine vulnerability calls out something hard-wired deep inside us men, a sort of pre-historic, instinct type, desire to guard and protect; and when its coupled with an inner strength and courage, it acts as a multiplier, screaming out that this girl is one worth fighting for.
Vulnerability alone is a primeval call for the man to bring comfort; But when there is a dash of strength in the mix, its a call to fight both FOR and ALONGSIDE. And it’s super attractive.
(A word of caution: to the degree that this is powerful attraction, is to the same degree opposite if its faked or used to manipulate.)