I see this question regularly in almost every newspapers nowadays. The authors usually answer the question by listing some of the most demanding exercises but they tend to forget the most important – i.e. the best exercise is the one you actually do. When people decided to lose weight they often commit 100 percent and decides to run every day or follow an extreme workout schedule such as P90X. The problem is that they usually go in the wall after 1-2 weeks and then they are back where they started with no exercise at all.
Instead you should focus on exercise you actually enjoy doing and do so in moderation. 3-4 pleasurable walks usually gets better result in the long run than if you try to run a marathon three times a week.
About the myth on low intensity exercise and fat loss, remember the following:
Low intensity exercise is good for fat loss but high intensity exercise burn more fat in the same time and also gives more health benefits. The problem is that most people can’t exercise at a high intensity for as long as they can on low intensity. The turtle always wins against the hare in the long run.
Focus on exercise you enjoy and if you like P90X so go for it!