It really, really, really depends on the guy. I would say I notice weight first, then the face/hair, and on. Also, it depends where I'm looking when I see the girl. If I'm looking into the distance a bit and there's a girl walking away, my eyes will be lined up with her behind, so naturally, not by choice, that's where I'm going to look first.
I prefer women who are a little bigger. When it comes to what I notice first about a women, it depends on if I talk to her or not. If I am just walking by her I notice her legs and "bum". But if I actually talk to her I look at her smile and eyes.