Both can also be used depending on what you want to mean.

Populated means the total number of people in the country whereas populous means number of people per square kilo meter i.e population density. Let me explain further:

The most populated country has maximum number of people in comparison to all the countries of the world irrespective of its land or area.
Whereas the most populated country means the it has the maximum population per 1 square kilo meter which makes more citizens to live in less area. Or you can say it has the highest population according to its area Or is the most crowded country. A country may be least populated because it has say only 80 lakh people but is most populous because it has only 10 thousand sq. Km land where everybody has to accommodate.

Both have different meanings and will refer to different countries in the above sentence.
Hope it helped.