How does one choose the panel to present at a conference? I am looking to present my work at a conference. The goal being to get comments and get a paper publication. I have a choice to go on the last day with the chair being an authority in the field. Or on the first day with upcoming chair. Which would be better to help me both get good comments and still scout for post doc positions.
I think each choice has its pros and cons. But having said that, depending on how long your conference is, it's possible you will get the most attention, energy, and enthusiasm if you present on the first day of the conference. From my experience, often people leave early (esp. for long conferences, like those that last a week). On the other hand, it's also possible that people arrive late and stay for the rest of the conference. But from my experience it's usually the former. So I think you may be better off presenting on the first day. 

In terms of scouting for post doc positions, you can approach the professors/researchers you are interested at other times during the conference, esp. if they are also giving presentations. In that case, attend their presentations, raise your hand and ask good questions, and/or approach them after the session.