There is always a pumpkin, smoked duck, or clam and noodles dish you haven’t seen before. The way to eat well here is to seek out the small restaurants, on the edge of residential districts, with no English language signs, which appear to not rely very heavily on the division of labor and which serve not too many dishes. Bibim bap (shaken vigorously inside a lunch box, I might add) is like a fine risotto and the quality of cabbage alone makes Seoul a world-class city.
Particular restaurant recommendations are pointless, and in any case hard to track down. Just follow basic principles. The street food, by the way, is only so-so.
At one restaurant, as a kind of joke, I asked “What is best?”, not even expecting my English to be understood. The waiter became very excited and opened the menu to a page entitled “Best food,” which listed five dishes. I ordered two of them.
I see no reason to explore upscale dining here. For surprise and uniqueness, I am not sure the world currently offers a better dining city than Seoul. My most expensive meals are still falling below $20, averaging $10-$12, and they are occasionally below $5.