Besides that there were actual sub-saharan African powers in history? Mali, Axum, Songhai, the Sokoto caliphate, and others were basically great powers. But if you mean an African great power surviving to the modern day, Ethiopia or Somalia is probably your best bet, given that they have historically been pretty wealthy/powerful (even if they aren't today).
The biggest weakness of Africa (besides all the colonialism and stuff they had to endure for centuries), is that the concept of nation states never really caught on, and all great powers that survived into the 20th century were based on nation states (plus their colonial empires). Not that this is the fault of the sub-saharan Africans (read Guns, Germs, and Steel for more on why they got a raw deal), but without very many coherent nation states, most African countries have to focus primarily on internal issues. And a country focused mostly on internal issues is by definition not a great power. The only real nation states in modern Africa are Egypt and Morocco. In sub-saharan Africa only Angola, Swaziland, Tanzania, and maybe one or two others have been able to form effective national states.
But other than that, the Swahili coast has a decent chance of becoming a maritime power, since they have a lot of chances to develop a common language, common identity, and economic ties (three major defining traits of a nation state). But the biggest possible sub-saharan great power is probably a unified East African state (either centered on Ethiopia, Somalia, or Zanzibar). The geography of Africa doesn't lend itself very well to forming a large unified state (large unified states can pretty much only spread along flat plains and river valleys, see also: Canada, USA, Russia, China, India, etc.). Brazil and Australia are the only real exceptions to that rule, but it took a lot of genocide of the natives to get to that point.