As a personal trainer I am often asked what is the best exercise I can do? Of course they usually mean to burn the most calories. Most people are looking to lose weight and burning calories will help you lose body fat. However, I usually answer this question with, "The best exercise is the one you will do." I stilll think this is the best answer.
But in regard to which exercise burns the most calories here are a few that really get the heart going; jumping rope, running, swimming, kick boxing/boxing, rowing, cycling and cross country skiing. Do any of these appeal to you? If so, give them a try. If you enjoy doing it, you are more likely to continue. If not, try to find something that is fun for you. And one of the most important tools in regard to sticking to a physical activity is to find a friend to be your "work-out" buddy.
Enjoy what you do and do it with a good friend and you are much more likely to stick with it.
As with all of my fit tips, please check first with your doctor.