Yes, if you're restricted to Eastern Cape, I'd say staying in Addo would be the best bet.

If you could go a little further afield, you could visit Pilanesberg near Sun City, a couple of hours' drive from Johannesburg, where you stand a good chance of seeing the Big Five. See:
- http://pilanesberggamereserve.com/index.html and
- http://pilanesberggamereserve.com/be...-tourists.html