There is no such exercise.
Sit ups and other core strengtheners will do just that - strengthen your core. They are fine exercises and you should do them (as should everyone). But do them for the purpose of strengthening your core and not for flattening your stomach. There is no such thing as spot reduction. It does not happen. Instead, you lose weight all over.
Want a flat stomach? Starve yourself so that your body fat is very low. And then get plastic surgery.