I agree with Zach. MA, although they can provide some exercise, aren't always the best method to stay in shape. That said, certain MA are usually better than others. MA that are more sports-oriented tend to provide a decent workout. Things like WTF TKD, Judo, BJJ, Muay Thai, or Kendo would usually be good at getting people in to shape.
Additionally, as most of the above systems are considered sports first and foremost, there is usually a lot of sports science to help you form a workout program. I personally have seen very good sports-specific strength and conditioning programs for WTF TKD and Judo. I've also seen very good stretching programs for TKD and BJJ. So if a person takes part in those sports they can often build an entire exercise plan around them that will help them in said sport while also helping them stay in good shape.
On the other side of the coin, sports-oriented MA, especially full contact MA, usually have a higher risk of injury rate compared to MA that don't focus on the sports side of things (in my experience). So you have to consider that too.
"Let your food be your medicine, and your medicine be your food"