Is there an ultimate diet for a woman to have a flat stomach?

No specific food: Even the most toned abdominal muscles will not reveal the abdomen you want if they are buried under a beer gut. Decreasing body fat thru eaiting healthy is a key component of washboard abs. That said, anything that strengthens your core (sit ups, crunches, leg lifts, torsion, etc ) all play a role in building those muscles, especially if you've had a baby. Rarely a tummy tuck is needed.