I don't like to stay for a long time at the airport on a transit, which is so boring. So I want to get out of the airport to enjoy the city if at all possible.
But many major hubs are so far away from the city centre that you won't get any decent chance to enjoy it.
For example, from Incheon Airport to Seoul station, it takes you 43 minutes even on express rail. In Hong Kong's case, it takes you 24 minutes from Hong Kong Airport to Hong Kong station. It's not so bad, but it still takes about an hour on return-trip, after adding the average waiting time of a next train.
I don't like to enjoy the "around the airport" atmosphere; it's the "center" of the major city that I want (e.g. Gangnam or Myeongdong, not Incheon, Shibuya or Roppongi, not around Narita; and I assume international airports).
Is there any international hub that is close to the center of the city, possibly within 20 minutes? I want to make my travel plan by taking it into consideration, especially when it comes to a cheap layover ticket.
I don't mind a country or city as long as it is an international airport.
Sorry, in international airport, I meant any airport where there is significant connecting traffic for international flights. I didn't mean just any airport that serves any international destination, which is the technical definition.