If your goal is sudden weight loss, do not do the master cleanse. You simply will not make it, you will not be adequately prepared; and asides, the Master-Cleanse is designed for cleansing - and while you lose weight, that should not be your focus on the MC.
The fastest method for losing weight (especially fat weight) is the Atkins Diet. Basically, the rules of this diet are simple: stop eating carbs. Eating less than 20 grams of carbs a day induces a state, which, if I recall rightly, is called ketosis, and means that your body is receiving insufficient carb energy to operate, so it begins to burn ketone energy, aka energy from body fat.