I recently participated in ACE’s new Group Exercise Leadership Specialty certification and felt like they offered a definitive answer to your very question! The best exercise to reduce “belly fat” (to be specific to your question), is ANYTHING you are willing to do a.) most safely b.) most intensely c.) for the longest duration, and most importantly (at least in my book), d.) with the most enjoyment so that you will want to stick with it for the long haul. I think that prescription along with a balanced and healthful eating approach–and all the great advice offered above–is a great start.