It depends what you mean by fastest really. There are dramatic things you can do with extreme diets and extreme exercise, but to me the best thing is to burn fat and gain muscle in a controlled, sensible and healthy way where you don't injure yourself or stress out your body or mind too much.
If you do a good mixture of cardio work like running, swimming, martial arts, skipping or anything else you like, and do some weight training sessions and maintain a good diet you should be able to achieve your goals in the best way for you. I am pretty fit and toned myself, and my secret is to do 2 2 hours martial arts sessions a week, 2 running sessions, and heavy weights 3 times a week for 45 minutes each sessions. I eat really well, with plenty of fish and vegetables particularly and drink gallons of water. I have lots of time in my life so I can do this much exercise, so it's not a problem. But even if you have less time, it's how you spend the time that is important. Make every exercise session count, and make every meal healthy!