ultimately, any form of aerobic activity that you enjoy and will engage in on an almost daily basis. Interval training within that activity is enjoyable and modifies the intensity, thus enabling you to expend more energy overall.
I also recommend that you add some resistance training with the goal to build more muscle which – in turn – will help you maintaining aerobic activity for longer periods of time and will create tissue that stays metabolically active when you are at rest, thus increasing your basic metabolic rate.
One last word: while you may want to target body fat in the abdominal region there is no exercise that will reduce the fat in that exact spot. You will have to leave it up to your body to determine where it will go to get the resources for the aerobic activity. But as long as you expend more energy than you consume, the fat will gradually disappear from the desired region as well.
I wish you good luck.