Africa is known for not having a lot of Esperanto-speakers. Still, which country does a good job at it?
Measuring numbers of Esperanto speakers is notoriously difficult, but UEA does publish some enlightening figures about its member countries, including African ones.
Judging by UEA members, the most active country is Togo, with 28 UEA members.
Judging by members of the national organisation, Benin is on top, reporting 500 members of the national Esperanto organisation.
How exactly this translates into Esperanto speakers is difficult, but in general the UEA figures give at least a good relative perspective on Esperanto in Africa.
As some numbers have already been provided and as was mentioned, official numbers are quite difficult to calculate, I'm going to first mention some personal experience. Most African Esperanto speakers I have chatted with online have been from the Democratic Republic of the Congo, Togo, Burundi, and Benin. I have recently met one person from Nigeria.
There is also the recently-launched Aminda Radio Esperanto, out of Madagascar.
On edukado.net, Congo-Kinshasa (Democratic Republic of the Congo) is the first African country I see, with 18 users listed as being from there. Next is Togo with 14, next are Senegal, Burundi, and Algeria--you'll find more countries listed as well as the numbers of users on the linked page.
Lernu also keeps statistics of its users' country of origin. I am pressed for time now as I write this and will try to find that info soon.
Edit: I am having trouble finding a page of Lernu's stats all laid out in one spot since Lernu's site redesign, but here is where you can search users by country, language(s), and gender. You'd have to go through each African country to see the numbers this way. I contacted the Lernu team to see if they can link me to a statistics page, if one still exists, but at least this way you can still gain some idea.