I wrote a hub on this, well it says 6 pack, but the routines will help you cut weight. You need to work your whole body, there really isn't one "best exercise". You cannot just do cardio you need to mix cardio with resitance training as well. Have a look at my hub, if you find that it doesn't help let me know and I'll send you some really good workouts!
Hope to hear from you soon!