That depends on different factors like what they're wearing, how much of them I can see, how tall/short they are, how much their clothing leaves up to the imagination or not, and if I even care enough to pay attention.
Most of the time I don't even look at people really closely, they're just sort of there.
But assuming I can see all of them then I look at all of them all at once. But if you have a hair color or style I dislike, I wont finish looking. Same if you're too skinny (for men or women) or too flat chested (for women). But if I had to say what I focus on, it's either the mid-section or hair.
Also what color shirt you're wearing. That's also what I notice. Also skin tone. Basically your entire palette.
Meh, actually... I think I notice too much at once to answer this question. But I never actually meet many people worth noticing. Last person I met worth noticing was when i went to get a book at B&N and the guy at the counter doing check-out was pretty good looking, close to my type (dark hair, dark eyes, olive skin tone, clearly older than me - maybe late 20's, was wearing a decently looking burgundy shirt... and commented on how he liked Edgar Allan Poe also - that last part really got my attention). But he seemed to be a bit on the thinner side, judging by his overall frame, but I also couldn't see all of him either.
Dammit... and that was last November I believe. Yeah, I really haven't noticed anyone since then. ****ing dammit, he was at least 75% my type too... physically at least.
Anyway, I guess I just notice a lot at once, sometimes a certain physical aspect will catch my eye and that's what I notice first because I just happened to see it and it suited my interests. So in a manner of speaking I notice whatever catches my interest about a person physically. If you really have nothing physically that interests me, I probably wont even register you.
Voices count too, by the way. A voice can catch my interest, even if it comes from someone I'd never look twice at if I had the choice.