What is the number one doctor-recommended exercise that leads to loosing fat around the belly?

Cardio Mix. You cannot target specific areas for weight loss - if you have adipose tissue to lose (not everyone does), then consider an increase in cardio & a balanced diet. However, do not always do the same exercise or you will increase your risks for stress injuries. Mix things up with walking, running, biking, swimming, or circuit training. Ultimately you need to burn more than you consume.