Usually the head, but in my practice sessions I just start from anywhere, from the leg up, from an arm, groin, feets... As long as you keep the proportions right there shouldn't be a problem to where you start. In fact, starting your drawings from a different section and working your way from there is a nice way to practice your overall proportions.
Also shoulder and hip lines are good indicators of the pose of a body, having them right is a solid step into building a nice figure, so that's why some peeps start from there XD.