That's a very hard question cause there are probably as many answers out there as there are guys.
I realized that I should give up trying to make all girls be attracted to me cos that's not possible - its a fact of life. Better to just be who you are- making a effort to stay fit and healthy helps too tho. BUT that aside...
For me, the eyes.. totally, hair too, sense of humor, and a feminine chick (like GoodManDave pointed out). Thats what does it for me. And then again, its just really the personality cause if I like the personality, the rest sorta grows on me.
I've said it before and I will say it a thousand more times. I love a girl who is a mystique. Sexiness is something that flow on someone, not forced. Wearing something that shows a lot of skin is not sexy; it's smutty. Leave some to the imagination. Intelligence is sexy. I also believe that as long as a girl takes pride in herself, it's sexy. You can't help what God gave you, but you can enhance it by keeping your body clean and healthy. Hope that helps.