That is a tough one to answer as south Africa has over 2500km (1553miles) of coastline. It really depends on where you are in South Africa, but you can check out, http://www.southafrica.info/travel/surf/beaches.htm ,
this gives a breakdown of the beaches by province. about 6 years ago
I would have to say Camps Bay Beach (socialite central), Llandudno Beach (blue-blue freezing cold water), Fish Hoek Beach (warm water), or if you're into the whole nudist vibe, Sandy Bay Beach.
Remember the sunblock! Especially for Sandy Bay :-P almost 6 years ago