Depends on the individual cichlid and what you consider aggressive.
But on average...
Dovii's can do the most damage, they are built to destroy. They are always prepared for a fight. Though I dont think their really aggressive, they are more territorial than anything.
Red Devil's are highly aggressive to the point where they can kill.
Umbee's are just big giants that love to chase smaller fish, just like green terrors.
Buttikoferi's are usually out going, but only if their the alpha(dominate) one of the tank. Otherwise they will try their very best to get up top, that means fighting non-stop. They are african, but are american inside
Black Nasties are probably the most aggressive American cichlid, they kill other fish as a hobby of their own. They are down right black demons.
The most aggressive cichlid of them all IMO are the mbuna's. Auratuses, chipokae's, kenyi's, and ect are blood thirsty killers. I've seen 4 inch auratuses chase 6-8 inch dovii's forever in my LFS. It's quite sad. Mbuna's are soo small, but are a handful that is packed with murdering instincts.