Have kept quite a few different Cental American, and South American cichlids over the years. Im gonna have to say the most aggressive pound for pound would be my Cichlasoma festae....TRUE RED TERROR! Had a pair that produced regularly, and were just psychotic killers.
The most aggressive fish I've owned so far was a Male Freddie.
He terrorized all his tankmates enough to earn a tank to himself.
Tankmates included Male and Female Festae, Salvini, Jack Dempsey,
Green Terror, and Convict (all males and all fish were the same size).
My Salvini is pretty nasty as well.
I can't imagine the damage a 12" Salvini could do
In my opinion the most aggressive cichlid overall is the Black Nasty!