Compared across countries

How do you get from the airport into the city?

For a trip that crosses borders — the same question, answered country by country.

The pattern

Rail is the answer in five of the six, and it is usually the cheapest as well as the fastest. Vietnam is the exception: no airport railway, so the number to plan around is time rather than distance.

Japan

By train, nearly always. Which train depends on the airport and on which side of the city you are sleeping.
China

Rail, in every case. Each major airport has a dedicated line, and the ordinary metro is always the cheap way in.
Thailand

From Suvarnabhumi, the Airport Rail Link on floor B1 — or a metered taxi, which adds a THB 50 airport surcharge on top of the meter.
Vietnam

Metered taxi or a city bus, and the answer scales with the city: Hanoi's airport is far, Saigon's is close but slow, Danang's is practically downtown.
South Korea

Train, bus or taxi. The airport railroad runs from both terminals into central Seoul and is the cheapest of the three.
Taiwan

The Airport MRT. One line, two kinds of train — take the Express, not the Commuter, and it runs straight to Taipei Main Station.

Sort the practical stuff before you fly.