Hi I've lived in Seoul for over 3 years and can tell you there are a lot to choose from.
Insadong Area has some really good traditional Korean foods.
- Seafood Pancake -Go to Anguk station , exit 6, walk onto Insadong street, take a left at second
street and walk to the 2nd or 3rd restaurant and order the Helmul Pajeon.
- Smoked Sliced Grilled Meat - Sam Gyup Sal is one of THE main commond dishes to try but there's a franchise that's put a twist on it. They smoke it first. Order the first 2 meat items on their menu. Go to Jonggak station, exit 5, walk on the side that has the Lotteria fast food joing and go inside alley streets, then take a left at the first intersection. Donenu is the name of the place and it's on the left hand side.
- I could go on, but not sure what your tastes are. Street food can be great too as well as some of the major market beer foods near Dongdaemun.
Hope that helps.