Great question!  Depending on what you like to do, I would stick with your favorite exercise because if you pick something as strenuous as sprinting to lose the most calories, you may not stick with it.

Great question!  Whenever I am performing an exercise and do not know the answer, I use a site called www.FitClick.com.  This site provides a calories burned calculator; you type in the exercise and then amount of minutes and it will provide you with an amount of calories burned.  Head on over and type in your jumping jacks!  Good luck!