South Africa - When To Visit
That depends on what you want to do.
The flowers are obviously best in August and September.
The best time for game viewing is late spring (September and October).
The southern right whales hang around off South Africa's coasts from about mid-June to the end of October.
The diving is best in most of the country outside of summer (ie, from April through September), and so is the surfing - but that certainly doesn't limit either activity to those times.
River rafting is better in the Cape at the end of winter, and in KwaZulu-Natal in the height of summer (late November to mid-February). In Mpumalanga and Limpopo, it's less time-dependent.
The "shoulder" seasons - spring and autumn - are best for hiking, as summer can be hot over most of the country. In the Drakensberg, summer thunderstorms are dangerous, while there is a good chance of snow in winter. In the Cape, the winters are wet, so hiking can be a bit hardcore.
If you're a birder, the palaeoarctic migrants arrive around November and the intra-African migrants usually by mid-October.
Of course, if you want to lounge around on the beaches, mid-summer is the best time - though everyone else will be there too. And - big bonus - the beaches of northern KwaZulu-Natal are warm and sunny even in midwinter.