One that makes your sweat. One that gets your heart and blood pumping. One you can feel the day after. Running is an exercise that does all those things!
High intensity training where your heart rate is elevated for a time and then you rest till it comes down. Do this repeatedly. Studies show that this type of exercise keeps your metabolism going for longer than just aerobics.
Something fast paced with always be a good fat burner. You dont burn fat until after 10 minutes so do double this or more to see results. Climbing is a slow exercise but is the only sport i know that exercises every muscle in your body, so as well as fat burning you will build muscle too.
See since i have a lil stomach weight i like to work on my chest and stomach so i can put on fitted dress and not look pregnant