Is walking on a treadmill or walking outside best?

Depends: Treadmill is even surface with consistent temperature, etc. Unfortunately people bore of this easiy whatever keeps you exercising is best.

Either is great.: Assuming you're wearing adequate footwear, walking is a great low-impact aerobic exercise. Walking outside is great for variety and fresh air, but just be aware of your surroundings and stay safe. With a treadmill you can listen to music or watch TV to keep you going longer.