What's the best diet for a obese person? how do you lose weight fast?

Nutrient dense foods: Eat foods that are nutrient dense (like meat, fish, poultry, eggs, cheese, low starch vegatables and water) avoid foods that are not nutrient dense (like starches and sugars, including fruit) nutrient dense foods will increase your basal metabolic rate. On the other hand, starches and sugars impair thyroid metabolism via affects on your liver (which helps activate thyroid hormone).

No specific diet.: Fast is not best. To lose weight insure that calories burned > calories consumed. Drink at least 8 - 8 oz glasses of water per day. Try for 8 hours of sleep/night. Aim for at least 5 servings of vegetables & 4 servings of fruit per day. Use whole grain when possible. Avoid sodas, alcohol & junk carbohydrates. Get in a good mix of aerobic & weight lifting exercises.