Unquestionably the Okavango Delta in Botswana and the Kruger National Park in South Africa. I have been to both and both are utterly wonderful with abundant game and great facilities. Okavango has a slight edge because of the waterways and the fact that one can walk in certain areas with guides.The ten days I spent there were some of the most enjoyable of my life. A Hub is in the works on it.
I have not had the opportunity to visit the Masai Mara but I do understand it is also wonderful.