Can't go wrong with either of the restaurants in the The Codfather Village (http://www.thecodfathervillage.co.za/village.htm)
Depending of course where you're staying. Some great spots in Melville as well.
Firts Night in Jozi ever? Head out to Melrose Arch and choose from a variety of good eating establishments.
If you are staying in the Sandton area, be sure to go to Mandela square. Lots of options. From local cuisine to meat, seafood, and even a Greek place... My favourite is the Butcher shop. great aged meats.