Which country today has the largest population?

China.

The People's Republic of China, also known as Mainland China or often simply China, is almost universally considered the country today with the largest population. As of 2012, population estimations are approximately 1.35 billion people. This is larger than any other country, the next most populated being India (with approx. 10% less: 1.21 billion). Therefore, China is the country today that has the largest population.