Tricky question.. when I'm talking to a girl I guess I look at her eyes first and the most.. then her mouth/teeth/smile, and probably her hair after that. At the same time though if I'm talking to her I probably check out every feature on her head within the first 5 seconds so it's pointless to say which I check out first. The top 4 non-sexual things I always try to see of a girl first though would be her smile, hair, eyes, and then her clothing/style.
It really depends on the girl. It depends on what she's wearing, what she looks like, what she's drawing attention to, things like that. I might look at her face, hair, clothes, body, her body language, what she's doing, or whatever else I happen to notice first.