If you're in Joburg, I strongly recommend the Local Grill which despite being a mid-market chain still has done hands down the best steak I've ever had (make sure you have their Greenfields rather than grain fed):
http://local-grill.co.za/

My second favourite in Joburg was always Wombles which really looked the part and do great on the bone specials but in my view wasn't quite as good. Compared to those two I found the Grillhouse meat lacking flavour.