I would put the ideal focal length somewhere between 85mm and 135mm with 35mm, 135mm and 180mm with medium format, depending, as said, on the subject´s face and framing, if you want to keep a somewhat intimate and not to much compressed look. I really think the facial proportions do matter in this regard. I have seen many portraits of children made with an 80mm in medium format (which is about 45mm in 35mm) and they looked great. Portraying an adult with a rather large nose with an 80mm lens on the other hand is a real crime.