Walking
 

It burns a lot of fat and almost no muscle.
Walking is a low intensity exercise, which means it burns a higher percentage of fat. True, walking for 10 minutes doesn't burn much fat, but walk briskly at an incline for 4-8 hours a week and you'll burn a significant amount of fat.

The fact that it doesn't harm muscle is probably the biggest aesthetic benefit to the T Nation crowd. High intensity exercise, particularly cardio, uses glucose for fuel. Normally that isn't a concern as the body will break down its glycogen storage (stored carbs) for glucose.

If on a diet and lifting weights, glycogen stores are more easily depleted. If you add intense cardio on top of this, the body will release cortisol to help convert amino acids into glucose to be used as fuel.

Those amino acids can come from your hard-earned muscle tissue.

Clearly, this is a problem for a lifter because whatever form of energy storage you have, you'll burn more of that particular energy store. Most people have considerable body fat, and the body is quick to burn that off once they get moving.

But a muscular and moderately lean individual will have more muscle than fat. The body will see the muscle as "excess" and will preferentially burn that muscle to meet the caloric demand of the exercise.