This depends on what you are looking for exactly. Countries are often ranked in size, population, GDP per capita, standard of living, life expectancy, etc.
This answer to this question tends to fluctuate with time. Here is one list of the top ten smallest countries in the world by area in sq. kilometers/ miles: Vatican City .44 sq km/ .273 m Monaco 1.95...
BY LAND AREA 1. Russia 2. Canada 3. America 4. China. 5. Brazil 6. Australia 7. India 8. Argentina 9. Kazakhstan 10. Sudan BY POPULATION 1. China 2. India 3. America 4. Indonesia 5. Brazil 6....
A recent study conducted in England shows that the top 10 air polluting countries are: 1. China 2. United States 3. Mexico 4. Russia 5. India 6. Japan 7. Germany 8. Canada 9. United Kingdom 10. Italy