South Africa= around 55,000,000 of total population (based on 2015 data)
Nigeria= around 185,000,000 of total population (2015 data)
Angola = around 25,000,000 of total population (2014 data)
Congo= around 82,000,000 of total population (2015 data)
Given that there's not an high population growth or birth rate or something like that, I'd say Nigeria