I have been to this restaurant many times over the past few years, and every time I visit Medan, I must go to Dainang. Try to get there early to beat the lunch/dinner rushes because the place is small. It doesn't look like much from the outside so don't be deceived! The place has incredible food with very reasonable price.
Try "Babi Goreng", the crispy fried pork in a small plate for one person, this is my favorite. If you order "Nasi Putih" or the white rice, the soup comes for free. The roasted pork (Babi Panggang) is also excellent, mix it with the black sauce and it gives you awesome taste. Don't miss the 'Ikan Lele Goreng' : the deep fried catfish with a very soft texture, this small fish only enough for one person.
If you are looking for a local authentic bataknese food, this is the place.