If its a boy, usually eyes/face. With girls, I try to look at their eyes (but this fails some times...) I only trust people who look at my eyes when they talk to me, so I assume more people will trust me if I look at their eyes when I talk.
I'm usually walking and talking, or with a group of people, and it kind of differs from person to person/situation to situation. But I usually try to avoid looking at crotch, and chest too, if it's a girl. Sometimes I fail at that.
Because of the fact that I can not focus on one spot for too long, I switch between the eyes of the person. I try to respect people by looking them in the eye. (I like studying their eyes and seeing if I can tell the type of stuff they go through bye how their eyes act.)
Look down at the floor/ground, stare at the wall behind them, or gaze very slightly away from their face. When I do make eye contact, it's usually quite brief. Probably not the best way to talk to someone, but...oh well.