The most popular calibers locally in RSA seem to be the .308, 303, 30-06 and 270 and perhaps in that order. Surprisingly enough the 6x45 with 100 gr. bullets is very popular in RSA. Probably the same in Zimbabwe, but I have seen a lot of 300 H&H rifles in Rem 721 persuasion over there also..
As to my choices, just give me a decent caliber from 7x57 to 404 Jefferys or anything in between and I'll be as happy as if I had good since...
If I had to pick "one" all around African rifle it would be the either the 30-06 or the .338 Win. but that would be my pick for about anywhere in the world if I was barred to own only one gun, and one would suit me as well as the other..