This is an extremely hard choice. If i chose a small fish, say under a foot, it would be a waste of space. I would want something that gets huge, is extremely strong, eats a lot, and lives a long time. This would be a showdown between and Red or black pacu or a Arapaima Gigas. If i had to chose it would be a Pacu either red or black, probably black because they get bigger.
It would be a waste to get like a Green terror (although i love them) because i would never be able to see them because they are so small. Stingrays are cool, if you are keeping them in a tank when you can actually see them. Peacock bass are cool, when you have like a small shoal of them so i couldn't pick that. So Black Pacu it is!