If I had enough money I'd have a tank of each!
It's so hard to pick one - if I could start again with my current tank I'd have south Anerican biotope - I think for the standard size tank there is a better range of South American fish based on colours, size and temperament.
However, If I had a tank big enough I'd love to try African cichlids.
But I could also do an Asian riverine tank or a rainbowfish tank and still be a very happy man.