Walking, in my opinion.
If you're having trouble getting started with an exercise program, or if you've fallen off the wagon, walking can be a good place to start.
Walking, for example, provides many health benefits, ranging from stronger muscles and bones to increased mood and a lower chance of disease.
Second, it's really easy to get to. Almost everyone, regardless of age, fitness level, lifestyle, schedule, or geographic location, can walk.
Finally, walking is extremely "customizable." That is, you can add items to your walks to make them more enjoyable (such as listening to an audiobook), and you can easily include extra steps into everyday activities (such as talking on the phone and running errands).