It depends on what you are after really? South Africa is the obvious choice with Kruger being the standout destination.
Botswana is the place I would head to followed by Namibia though. You won't see the big five there but they are nicer countries.
If you like to rough it (not rough by any measure but a bit more spartan than the big resorts then I would head to Mana pool Zimbabwe. It's a national park and you will stay in tents but you can't get any closer to the animals.
The in laws know a camp owner up stream from Mana on the Zambezi and it's my favourite spot on earth. You will see everything bar Rhino and Giraffe, the Elephants will interrupt your breakfast, Hippo interrupt your evening Brai and you will wake up surrounded by lion prints as they sniff around you whilst you sleep!
This thread is useless with out pictures, these are few taken with my phone whilst out and about in the Zambezi valley.