It depends on the guy and what my relationship is with him. If we've only just met then I tend to notice eyes, closely followed by lips (and smile).
I have lusted after someone simply because of his hands as well though. I love watching men drive, I think it's something to do with the control (which probably tells you a lot more about me than necessary)
If sex is involved though then I tend to get distracted by that area of skin where the neck meets the shoulders, just above the collarbone. I don't know why, I just love to... nevermind.