I think it is Hainan. Why are you so sure? Because I have been there! Many people retire, especially Heilongjiang in the northeast, and some people settle in Hainan. The scenery here is really good, especially in winter. When it is freezing in the north, Hainan is still refreshing in summer.

People can do outdoor activities at any time. It's like this all year round here, although it's particularly sultry in summer. But the air is fresh and pleasant. So I think Hainan is the most suitable city for providing for the aged. The other line goes north to Guangzhou. Although there are many high-rise buildings and the traffic is developed, you can feel the fast pace of the city very well.

But compared with Hainan, the air quality is very different. The water quality is different from that of Hainan. Hainan feels good in all aspects, except the bustling metropolis, but the enjoyment of natural environment and outdoor taoyuan is something you will never experience in urban life.

Of course, there are other cities in the mainland that are also good, and you can choose according to your different preferences.