BoyPharoh
Google
One of Egypt's most frequented traditional restaurants by locals & foreigners. It has one dish only (1 soup, 1 dessert, soft drinks not considered) which koshary (mixed rice, lentil, pasta, tomato sauce, chic pea, fried onions, vinegar & hot sauce. A carbohydrates bomb that is so delicious.
The spot is so busy throughout the day, but especially evenings with tourists flocking towards it to try.
Koshary itself is delicious, but you can find better alternatives elsewhere. That said, a must try once when in Cairo.