It all depends on what you want to achieve from the road trip, if you want quantity with a few bigger ones here and there then go to Bloemhof, Vaal Dam, Harties and Donaldson. (And Demolition Dave has done really well at wriggleswade)
If you are going purely for the size of fish, to increase your PB then go to Doorndraai, Klaserie, Buffelspoort and Rietvlei. You will catch less overall but when you do, you stand a better chance of getting really big ones.
But your best bet is o do as much research on dams and their catches as you can, then you have the best chance of a successful trip.