there isn't really that one activity that is the best because if you don't enjoy doing it, then you won't be doing it consistently.
The best way to keep yourself fit is to find an activity that you enjoy and then do it every day, every other day, once or twice per week. However often your lifestyle allows you to do it. But it has to be something you enjoy. If you think jogging is boring as hell then don't force yourself into it. You'll do it for a week or two but soon you'll try to find excuses not to do it. You need to find the activity where you're trying to find excuses to do it.
That's the difference between doing sports to lose some weight (and then getting that weight back once you stop doing it) and doing sports as a lifestyle.