What countries in Africa were the majority of black slaves from?

Most slaves came from the region of West Africa. The country today where the largest number of them—nearly two fifths—came from is Nigeria. Nigeria didn't exist at the time of the slave trade. It was created by British imperialist conquest.
Most African slaves came from small tribes or stateless peoples. They were preyed upon by African empires, some of them building their economy almost entirely on slave trading. They were also preyed upon by Arab and North-African slave traders who forcibly marched them across the Sahara, in a death toll perhaps as high as crossing the Atlantic Ocean. The slaves that Europeans took came from the African empires near the coast who raided into the interior. The empires of Ghana, Mali, Songhai, and kingdoms of Benin and Oyo captured the most slaves for the trade. Another empire, that of Kongo in what is today Angola in Southern Africa, was based mostly on the slave trade.