Great subject, here's my two cents. If you go by aggressive display I think the Firemouth has it down pat. Red in the animal kingdom is a sign of danger, and the two fake eyes on the Firemouth gills makes others think it's fighting two fish. I think an Oscar's display is also pretty intimidating. They open there big mouth up really wide and do a lot of of bull rush movement. Oscars aren't really tuff compared to other cichlids but they sure jesture like they are. If you're going by just the look of the fish I would go with a Grammode.
they look like pitbulls with there big heads and muscular short body.