It's another person. why are girls different or are you saying guys think right while girls only think about shoes and shopping and how to deal with both thoughts at the same time... frankly you have given me the amount of info it would take to make a robot joke about how simple humans must be for any of them should react to any number of eachother.
Personally, (and I seem like an @$$ for saying it) I look first at someone's face, and then their body. A really, truly beautiful face, even on a fat/skinny girl, is most important. However, since my standards are so high, no girl ever passes the "truly beautiful" part, I look at their body, i.e. if it's athletic, curvy, etc. NOT SKINNY. I think girls that are like 70 pounds are, in general, ugly. Of course, if they have a beautiful face then it's okay(same in reverse for chubby girls, 'cept that chunky girls are better then skinny, generally). It's actually a complicated formula in my head, because in choosing my crush I factor in smartness, athleticism, face, body, and my realistic likelihood of going out with that person.
After reading this, you should probably realize why I never have any good girlfriends... :(.
I'm not a guy so this is just a guess - but I assume the laws of attraction work both ways, so I'd say the first thing any human being notices about another person is their physical appearance and each person has their own preference when it comes to what they find physically attractive. But anyone who says they see personality first is just trying not to sound shallow - the truth is it takes a while to get to know someone well enough to know their personality. So in an instant attraction scenario I vote that physical appearance is the first thing that will be noticed.