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.
First, realize that body fat is lost as a whole rather than just from the belly alone. Seeing as how the body loses that fat from everywhere at the same time, working all of the body at once is a great way to encourage fat loss. When muscles are being used, the body wants to keep them. When weight is an obvious burden, the body wants to get rid of it. You need to convince your body of fat being this burden. Total body circuits, intervals, etc. should work great for this if you eat appropriately.
Supportive nutrition, more strength, more speed, and ultimately recovery that will balance hormones. This means stress reduction, enough sleep, and so on. I have never seen and elite gymnast (one example) that had a lot of fat. People that are lean are usually strong, fast, and eat to look that way.