Is walking the best exercise?

[CrimsonEclipse]

I DNRTFA, but....

There is no BEST exercise.

It's a combination of what is around you, what equipment you have and the price to purchase, and what kind of physical shape you are in.

Walking is great if you have a safe place to walk, and have relatively healthy feet, ankles, knees, and hips.
Cycling is easier on the ankles and (usually) knees, but harder on the wrists, neck, and lower back.
Recumbent is easier on the wrists, neck, and lower back except you cant' ride out of the seat (off road) and you head is usually right bumper height
Swimming is excellent and easier on most joints, but safe swimming places are not universally available.

So, it's not what is the best exercise, but what is the best exercise for you and your location and financial situation.