Dramas with the best/drama defining OSTs

I recently finally got to watching Itaewon Class because I came to really like 시작 from the OST. Are there any shows that you feel like have a track or overall OST that is so good it significantly elevates the drama for you? Or perhaps is so good it because synonymous with the drama? Another example I can think of is Stay in Goblin.

I’d be very grateful for any suggestions and recommendations!