The one you will keep doing! Whether its running, biking, or swimming just go out and try it. Try to do it for at least 30 min at a time. If you're out of shape, then its ok to walk or go slow, just try and keep building up to that. I prefer running outside on trails as I like feeling the wind and thinking more about where I am, rather than just forcing myself to run on a treadmill looking at the same thing

I like to jog and run at the park trail with a few people to motivate me.. I also walk to the park as well