Very easy decision, go to Sodwana Bay. It could be argued that it offers the best diving in South Africa.
There are a number of diving operators based at Sodwana. I dive with
Triton Dive Charters as often as I can and I could recommend them very strongly.
If, for some reason Sodwana doesn't appeal to you, you could also consider some place in southern Mozambique. In the extreme south you have Ponta do Ouro, Ponta Malongane and the slightly more upmarket Ponta Mamoli but honestly, as far as I'm concerned these places are no better than Sodwana and it's just easier to get to Sodwana.
Further north along the coast you could consider diving around the Inhambane area (Guinjata bay, Tofo etc.) or Pomene. I have not dived the Inhambane area myself and hopefully some others will give you good info for that part but I have dived
Pomene and it was absolutely wonderful. Highly recommended.
Hope that helps, and feel free to ask away if you have further questions.
If you're interested to see some pics that I took at Sodwana and Pomene you can have a look at the following albums on my facebook page for some recent pictures:
Sodwana pictures
Pomene pictures