DS 160 form - list all countries/region visited in last 5 years

You were in China, but only in transit. Maybe the US visa officer doesn't care about that, but maybe they do. If they don't care about it, but you mention it anyway, there is no harm done. They might ask you some questions about the nature of your visit to China, which you would answer by saying that you were in transit.

If they don't care about it, and you don't mention it, there is also no harm done.

If they do care about it, and you do mention it, then once again they might ask you for some details, which you will of course provide, and again there is no harm done.

If they do care about it, and you don't mention it, and if they notice that you were in fact in China (which you should assume that they already know because you are on the passenger manifest for your flight from China to the US), you risk being found inadmissible to the US for deception, which is an outcome you certainly want to avoid.

Therefore, mention China in your response.

The same logic applies to the United States and to any other country you may have been present in, whether in transit, as a temporary resident, as an actual visitor, or in any other capacity.