I had a Jaguar who was a beast... tried a couple times to eat my Perruno Catfish!!! I had to get rid of the catfish. He was in a tank with a JD, Red Devil and Buttikoferi... they would all be pushed to one side of the tank. He even went after my Silver Arrow a few times. I bought him big and only had him for about six months, but he was pretty bad.
The most aggessive / relentless fish was my "Flying Dragon" Flowerhorn. At 7" or so he would fight anything and everything. He fought my Wolffish of about the same size. The FH was all tore up, but would not stop going after the wolf... even when the wolf was just trying to get back in his cave the FH would go in the cave to fight some more. The wolf really tore him up... I mean pieces of his lip and face were just hanging down. I was so mad at myself as I only had the FH for a 1 1/2 days. He got his own tank and would patrol the front constantly. Nothing could have ever been in there that he looked at as competition.