I didn't have a bike when I was in Africa, but I certainly found places worth riding, some of them already mentioned:
Sani Pass, Eastern Lesotho / Drakensberg Mountains. Note the hairpin turns.
From the top of Thabana Ntlenyana, tallest mountain south of Kilimanjaro in Africa. No iron pony, but my flesh and blood pony seemed to have carb problems. It kept surging, nearly throwing me over the pommel each time the surge stopped. IIRC, a reasonably skilled single track rider could get up here:
Somewhere in central Lesotho:
10 hours on the back of this, on these roads, will make you wish for a DS bike in northern Mozambique. This was shortly after one passenger was thrown off and broke his ankle, due to the blowout on a slick mud road:
The entire south coast of South Africa is worth seeing, and a beautiful ride. Hermanus, whale watching and great white shark diving capital of the country:
Of course, there are some places in Africa where you're better off having a bike with 4 wheels. SE of Swakopmund, Namibia:
And if you can get your bike through the Kaokoveld, there are some beautiful remote areas to see. That is, if the Namibian government hasn't turned it into a hydroelectric dam by the time you get there. Epupa Falls, Namibia, with southern Angola in the background:
I really want to get back to Namibia with a middle weight DS...