It should definitely be Nigeria. The Super Eagles might not be at their best now, but Nigeria has the highest number of professional players in Africa. If you take a headcount of African players who play abroad, you might ascertain that Nigeria has the highest population. Maybe the high population of the country contributed to this fact. Just like Brazil, Nigeria can boast of more football talent in Africa.
Another indicator of a football nation is the popularity of the sport at the grassroots. Football is the most popular sport in Nigeria and it is played by children in almost all the nooks and crannies of the country.
I support your choice, it's not because Nigeria is my home country but it's a fact, asides having the highest number of African players spread across different European leagues and other leagues too, Nigeria has produced a very good number of African best players if not the most so if they were not that good, they won't have been achieving that the most. Truth be told we have lots of talents that are yet to be discovered if only our government can look into our football sector, rebrand the national league then build more stadiums, make it more interesting and entertaining to watch, an average Nigerian male is lover of football but most folks who got talent don't pick interest to play cause we lack good amenities and facilities.