Most aggresive fish i have owned were my first 2 oscars.
I have kept JD's, a red devil, red tiger cichlid and a texas and those oscars were far worse which i find hard to understand
My hybrid firemouth X Convict is by far the meanest thing I have in any of my tanks.. It has been with oscars, firemouths, convicts, RT, GT, and an assortment of sharks.. killed a couple sharks, and many of the other fish it was with, he is now in a 150 tank with a couple siblings and a jack that is about 3 times his size.... and its starting to get bullied too!
I had a Jack a few years ago, and it was a glass banger. Every fish I put in the tank with him turned out to be a snack, and it was still a juvenile when I got rid of it. Right now I have a breeding pair of cons, and that Jack makes them seem like guppies.
well i have changed my mind .. my 5" blue texas has been remove from my big tank and put in a 75 .. he has managed to beat the living hell out of every fish he was with.. 6" male jag , 6" make cuban .. he almost killed my other 5" male jag,and female blue texas he is now growing a nochual hump almost overnite,and it is getting bigger every day