Facial structure. Though it depends on circumstances and the man in question. If a guy's walking around without his shirt on, it would be shoulders and back. If he has amazing hair or a foot tall mohawk, than obviously he's practically forcing you to see his hair first.
Second comes size. If a guy is smaller than me I'm not giving them a second look :P
THEIR EYES!!! honestly.
Though it depends on circumstances and the man in question.
If a guy's walking around without his shirt on, it would be shoulders and back.
If he has amazing hair or a foot tall mohawk, than obviously he's practically forcing you to see his hair first.