I know what you are going to say. You can't choose just one body part or trait, it's the whole package, but I beg to differ. The thing that most stands out when I look at a cute boy or girl is their eyes.
Since eyes are the windows to the soul, I get a great reading from a person by looking directly at him or her. Do you ever notice that someone's eyes can actually smile at you, warm your heart or completely turn you off? What do you find most attractive about the opposite sex?