In my opinion, walking is the best exercise one can do, running comes second. The reason for that is found in evolution.
In prehistoric times, when Homo Sapiens was developing from the missing link, he lived strictly a nomadic life. He walked when he could, ran when he had to (to pursue a prey, or to run away from danger). So human body was evolved to walk and (to a lesser extent) to run. So in my opinion, walking is undoubtedly the best exercise, it beats anything else by a mile.
Besides, walking has one big advantage over running and other exercises. If you overdo walking, there are no adverse effects, your feet may hurt a bit the next day, and that is it. In any other kind of exercise, if you overdo it, there could be serious adverse effects. Also the possibility of injuries is minimal in walking. It is very real in may other exercises (twisted ankle, pulled muscle, tendonitis etc.).
So without doubt, walking is the best, running second.