Not a Central or South American, but I'd go with Lamprologus ocellatus. Tiny shell dweller from Lake Tanganyika (around 2"). Fiercely defends its fry. Bit me a few times, and it hurt. LOL! Couldn't imagine what one would be like if it grew to a larger size like some of the bigger cichlids.
Do you think a adult at 6" Dovii could dominate a Convict or a RD? surley a Dovii at that size would be like a Salvini? although i have never kept a Dovii and only seen them in a aquarium in Tenerife in the canary islands
Not dovii. Red devils are meaner ime. Convicts are def up there pound for pound. 5 star generals are up there too. But honestly I've had a channel catfish go through cichlids like toilet paper, only to be sucked half to death by a pleco. The world is a strange place.
Also toughest and nastiest are different attributes. It's tough to find a fish that can take a whippin like an Oscar, although they aren't really considered nasty, I would label them tough.