Because Korean meals are not served in courses but everything all together at once so it's kind of hard to say which dishes are main dishes but I think it's safe to say these include protein dishes made from beef, pork, chicken and seafood. Main dishes in Korean cuisine are dishes that is the star of the meal, served in large platters, sometimes served sizzling or boiling in pots.
If you want to try one recipe from each sub category, try Bulgogi from Beef, Korean Fried Chicken from Chicken, Pork Bulgogi from Pork and Spicy Squid Stir Fry from Seafood category.