Burn the most calories - elliptical.
HOWEVER - most of these calories would be from existing glycogen and carbohydrates reserves, and not from fat.
To burn fat effectively you have to exercise for at least 30 minutes.
listen to me. The elliptical burns the most calories, not the treadmill. Mostly because of the ability to go in reverse. This target a completely different set of muscles. You can also do this on a bike but you do not get the upper body motion that you get with the elliptical.