Speaking in terms of aggression, I'd say a Red Devil. Many people say Jags, Oscars and Doviis, but they have a wider array of temperaments. A RD is initiating fights like 97% of the time.
I think in it's weight class,the flowerhorn is the most aggressive.The only fish that coexist w/ mine are convict cichlids.As a matter of fact one tank w/1 flowerhorn and 1 convict have bred.The fry are only1/3" right now so I dont know how they will turn out. But anyway the F.H.s kill anything else they come in contact with, thats why only 2 fish per tank. My oscars have big mouths but they're slow. My jack dempsy is only 4" right now and peacefully co-existing w/ about 14 other cichlids right now.Maybe mine just have a bad attitude but my 1st F.H. gently eats food from my fingers.
MY flowerhorns kill anything but convict cichlids which mine are paired up with. One flowerhorn and a convict have mated and now I have a smaller tank with their fry in it. Looks to be about 200 of em @ 1/3" now. Too early to tell how they will turn out.My oscars are bigger w/larger mouths but too slow.My jack dempsey gets along fine w/ 14 other cichlids w/him. This may change later when he's bigger.He's @ 4" now.In it's weight class, my money's on the flowerhorn. One flowerhorn gently eats food from my fingers.