I have found yoga to be a really good way of keeping the weight off – had to lose weight another way (combination of diet, weight training and cardio) but then yoga was a great way to prevent gaining the weight back.
Yoga is not just exercise but a way of life. So, I believe the answer would be Yoga. In fact, Hatha Yoga also emphasizes diet — what to eat and how to eat — as an essential part of the practice.