The meanest fish in the small's would be the Jewel cichlid hands down. In the Mediums would be the Red Devil or Midas and in the Bigger class id vote the Dovii and the Jaguar cichlid however some other perty nasty cichlids would be the Red Terror, ive seen nasty Oscar's also, ive seen evil Jack dempseys and as for africans, all of those have some attitude also.
Jewels are not nearly as nasty as Aratus/Chipokee or Kenyi. Sorta irritates me that petco sells like 4 species of africans all in the same tank... Yellow Labs (not aggressive at all) Red Zebras (who are actually orange, and are moderately aggressive) Bumblees (on the higher side of aggression) and Kenyi (super evil) Ummm... hey just a thought chain stores why not sell something that people can actually keep in harmony with out mass issues.... ok ok
JD's are South American btw... SA's are way meaner in some respects, but are less nippy. Africans peck you to death slowly, SA's beat you to death in a minute.