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The way we measure most populous cities / most dense cities is weird, and hinges on arbritary factors (take, for example, Chongqing, the "most populous city", which is mostly rural land, in a "city" the size of Austria)
I think a good metric that captures the population / density of a city is the number of people that can be reached with half an hour's or an hour's worth of transportation (1/2 hour down and 1/2 hour back is one hour both ways, a very common commute time, though a radius of 1 hour each way still contributes to the connections available) - this does have the effect of counting a larger area for areas with better transportation, but I think that's a good feature of such a metric.
This metric would remove any arbitrary influences caused by arbitrary boundaries, which is needed for good, meaningful comparisons. I would very much like to see a list organized by this metric.
related map of the US, with clustering of actual commutes: https://www.atlasobscura.com/articles/here-are-the-real-boundaries-of-american-metropolises-decided-by-an-algorithm . Note this uses longer commutes than I'd ever consider.
(edit: removed stray period at end of URL)
1) Population density has an important impact on the mileau and opportunities that exist in a given location, but we can only make meaningful comparisons when metrics are standardized. 2) I've heard it said that in medieval times, many lords would collect a "bushel" of taxes from the peasants, where the bushel was measured in a large basket, but then when paying a "bushel" of taxes to their king, the bushel would be measured with a much smaller basket, thereby allowing the lord to keep a larger amount of grain for himself. When we don't have consistent standards for metrics, similar failure modes can arise in (subtler) ways - hence why I find reliance on arbitrary definitions of location to have bad taste