I’ve tried a good number of roasters on this list, and only a few come within batting distance of Father Coffee (not to be confused with Fathers) in Johannesburg and Rosetta Roastery in Cape Town. Those two roasters, the former more experimental and the latter more traditional, are doing wonders and punching well above their weight in the South African market. Both could easily be in the top 25, and Father is surely top 5.
Seriously, if you care about coffee and find yourself in Johannesburg, go to Father and grab as many beans as you can carry.