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Athletics has been the most successful Olympic sport for South Africa, contributing significantly to the country's medal tally. According to the search results, athletics has produced the highest number of medals for South Africa, with six gold and 11 medals overall. Other sports like boxing and swimming have also been strong contributors to South Africa's Olympic success. For instance, swimmers Cameron van der Burgh and Chad le Clos have contributed to the country's medal count with their performances in the pool. However, athletics stands out as the sport that has historically brought in the most medals for South Africa. For more detailed information, you can refer to sources such as Brand South Africa and Wikipedia.