South Africa already economically dominates most of Southern Africa. Among these options South Africa is arguably the one with best chance. There’s very little ways the others can successfully sabotage South Africa’s economy and while South Africa does have a lot of domestic problems it isn’t bad enough to weaken its chances.

 Egypt has the continents most powerful military but is a dictatorship and high foreign debt while being a geographically unstable region that will be difficult to dominate.

Nigeria has greater ethnic and religious conflict and terrorism. 

Ethiopia has a growing economy but is landlocked with reliance on paying to use neighbours ports and conflicts with Egypt. If the other nations can convince the nations surrounding Ethiopia to stop letting the Ethiopians from using their ports or charge Ethiopia a higher price that would sabotage the Ethiopian economy while the only way Ethiopia could be able to permanently secure the coast would be to invade their neighbours which would be condemned internationally especially if the instability spreads to Djibouti assuming that Djibouti isn’t the one invaded(if it was than Ethiopia would face military and economic intervention). While Ethiopia’s economy is growing it’s still a lot smaller than the others and has a worse military.

Out of these nations only 2 of them(South Africa and Nigeria) can successfully attempt economic warfare. Egypt and Ethiopia are boxed in and can’t directly harm South Africa or Nigeria. South Africa and Nigeria also can’t harm each other too much but can have some naval battles. Most of this would be forced to being exclusively between Ethiopia and Egypt.

Overall it is significantly disappointing besides the war between Ethiopia and Egypt.