Which type of cardio exercises produce the most favorable results for fat reduction? circuit or hiit?

Any exercise will do. Do the aerobic exercise that you like. Exercise enough to induce perspiration. Exercise at least 150 minutes/week. You will need to reduce your food intake to lose fat.

Join a gym and seek out a trainer for a cardio program. A treadmill and bike riding and even a spinning class will give you a good workout. A diet low in carbs will also help as well as drinking a lot of water.