some will say eyes, others will say breasts, or skin, or mouth, or butt, but the one that every guy notices whether he says so or not is confidence. You can be attractive and 200 lbs if you are confident and keep yourself well, and you can be worth passing by at 5'7" , and great legs. It's not about what he looks at, it's about what he notices.
I think it really depends on the guy. Most guys notice everything at once, the whole package.
I do the same too, though, I usually for that split second notice the face first. That isn't to say I'm looking to see if she's "hot" or "cute" or "ugly" but to see if I like the way she looks. Usually when a guy finds a girl he thinks is attractive, most of his friends will respond like, "Uaghhh dude she is not hot."