Which foods do doctors think are best to eat to get a flat stomach?

Its not 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 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, so eat healthy and don't expect a magic food to do your hard work for you.