I suppose it depends on how you define luxury, but I do agree with the previous poster that MalaMala is a great value for experience product. The location of the MalaMala reserve is key - right up against the western boundary of the Kruger, the reserve is away from communities/fence lines and the Sand River runs right through the property attracting wildlife especially during the dry season. MalaMala is arguably the most productive wildlife viewing in the whole of South Africa. MalaMala Main Camp starts at $985/person/night for a Luxury Room. Luxury Suite is $1020/person/night. Another good one is Kirkman's Kamp by AndBeyond. They have a great area and have traversing rights into the southern sector of MalaMala. Value for money here is even stronger than MalaMala Main at around $755/person/night.
Finally, I'd check out Mashatu in southeastern Botswana. Accessible from Johannesburg by either a commercial flight to Polokwane (PTG) + road transfer or a scheduled light air on Angel Gabriel from Lanseria (Johannesburg) directly into Mashatu at Limpopo Valley Airfield. Known as the Land of Giants, this place remains one of the top destinations for big cats. Lots of active safari options and exceptional wildlife viewing. Mashatu Euphorbia is their new 5 star product and goes for $860/person/night.
Good luck!