In a sense, the question itself is problematic because importance is not well defined. Moreover, it's quite difficult to predict the future accurately. It seems likely that each will be an important state within its own region -- but North Africa, Central Africa, and South Africa are very different geopolitical regions.
Nigeria, due to its extensive oil and mineral reserves, has the potential to become quite wealthy, but whether that wealth leads simply to a small kleptocracy getting rich or to wider development depends on its ability to combat the currently endemic corruption. South Africa has a core educated populace, a tradition of democracy, and a strong educational system, which, if problems such as factionalism, HIV, and economic inequality can be addressed point to an ability to establish a broad economic future not based solely on extractive industries. The future of Egypt depends mainly on its ability to successfully manage restoring political and economic traditions of success in the wake of the Arab Spring.