Seoul

Seoul has about 25 million people. (The Chinese name of the Seoul-Incheon metropolitan area is 汉城).  But since I am splitting up the other major cities, I will split this as well.

These are the cities of note in South Korea

1. Gyeonggi Province (11.9 million, $289 billion)
2. Seoul (首尔) (9.8 million, $343 billion)
3. Busan (3.6 million, $76 billion)
4. Incheon (2.7 million, $71 billion) 
5. Daegu (2.45 million, $45 billion)