i don't think there is a single "best" exercise. It's more about what works best for you, both physicaly and mentally. For me, incline walking on a treadmill burns tons of calories without stressing my joints. I also find the Stairmaster to be the ultimate in calorie crushing. Find an exercise you enjoy and which gets your heart rate elevated (70-80% of max heart rate) for 30-40 mins and keep at it.