Which is the best exercise, not the quickest way, to lose fat from your abdominal area?

Abdominal exercise. While sit-ups can increase abdominal muscle mass they will only minimially help you loose stomach fat, if any. To lose adipose tissue work on a cardio mix (walking, running, biking, swimming, circuit training, etc - mix it up). Keep in mind that you can not target one specific area of the body to loose weight, and over time the adipose will decrease in many areas of your body (stomach included).