Population density is a metric often quoted, however it seems to me not always a useful tool.
The Netherlands for example, has few hills, or in general, gradients, and so most of the country can be built on (of course, they reclaimed some of it, and they could reclaim more!).
Contrast this with Hong Kong, which has got quite a few hills/peaks (compared to something like the Netherlands! They ain't no Mount Everest, but a lot higher than the high point of South Holland of 37m).
I tried to find, but failed, to find a population density measure disregarding, or discounting, mountainous areas or other relevant geographic factors?