Which kind of physical areas would most likely have the highest population density?

In the USA....

The highest mountain ranges would have the lowest populations as would areas set aside as national, city, town, or municipal parks. For example, Yosemite has permanent citizens residing within its borders, except for wildlife.

The State of Alaska has low population due to cold and having no sunshine six months a year.

Next, the vast stretches of desert would also have low populations, except nearest major desert cities.

Further, areas known as the "Rust Belt" where former steel manufacturing has left the area or gone out of business, the populations have dwindled in small and large municipal areas.

Large towns, suburbs, and major cities would have the highest populations.

Outside of the USA

Many countries are similar to the USA, with lowest populations in mountain and desert regions.

However, Middle East countries are desert lands so their populations are be spread further through sandy lands than in USA deserts.