For me its kinda whatever 'pops'. A lot of girls have that one thing that immediately draws my attetion before everything else. Unforutnately its not always a good thing ie a moustache. Other things are amazing like green or hazel eyes
All in all for me it will never be the same for each girl because all girls are different :)
Before my eyes focus on a girl and I become aware of her, I've already subconsciously noticed something in my peripheral vision that is her best feature. By the time my conscious mind comes around, that's what I'm looking at. Could be her butt, her hair, her face, even her clothes.