If you were to swap countries and imagine that Japan had dropped the bombs on the US, what cities best stand as substitutes? Would dropping the bombs on Hiroshima and Nagasaki have been closer to demolishing Los Angeles and Chicago (the 2nd and 3rd most populous cities in the US)? Or would it be more analogous to Silicon Valley and Detroit at its economic pinnacle (two incredibly important economic centers)?
It depends since the populations of both countries were quite different. So we could do this from a percentage stand point or a pure comparison of population standpoint.
Nagasaki and Hiroshima though were not large cities then, and are not even large cities now. They are on upper medium sized city in terms of population. So an equivalent would be any city with a population between 200k and 500k.
In today's Americans cities that would include places like Dallas, Texas or Honolulu, Hawaii.
Things like population density would also have to be taken into account though when thinking of a nuclear device since come cities are very spread out and others quite tightly packed.
In a report from 1945 that was meant to give Americans a sense of scale for the bombing campaigns, the US Army Air Forces said that Nagasaki was comparable to Akron, Ohio, and Hiroshima was comparable to Seattle, Washington. Neither were extremely populous by either Japanese or American standards; neither were anything like the economic powerhouses of Silicon Valley or Detroit. They were "mid-sized" cities.