Monaco is the world's second smallest country in the world and considered the world's most densely populated country. The total area of Monaco is 2.2 km (0.78 sq. mi) with a population of 37,831 in 2015. Population density is calculated as population divided by total land area. The United Nations stated Monaco's population density was 25,718.12/km.

Technically, not a country but a Special Administrative Region of China, Macao had a population of 584,420 in 2015 with a total area of 115.3 km (44.5 sq. mi). It's population density was 22,477.69/km.

Hong Kong, also a Special Administrative Area of China, had a population of 7,313,577 in 2015 and a total land area of 2,755 km (1,064 sq. mi). It's population density in 2015 was 6,654,74/km.

Singapore, an island city-state, had a population of 5,618,866 in 2015 and a land area of 719.9 km (278.0 sq. mi). Its population density was 8,226.74/km.

More Info: statisticstimes.com