Running or jogging if you are a fat bastard. People forget that when you are like 100 lbs overweight you jogging is like the 100 lbs lighter version of you jogging with 100 lbs of weights strapped to his body. Your knees, including both the muscles and especially the joints, were not meant to handle that kind of strain and you can seriously fuck your knees up permanently trying to do basic jogging to get back in shape.
When you are that fat you need an honest-to-goodness exercise bike to offload all of your body weight onto that eat (better be a good one, and preferably a comfortable one as well) while you are doing that cardio. (And you obviously need to go on a diet.)