I think with a 55 gal, you'll be pretty limited with the SA cichlid choice, expecially the excessively aggressive specises you like. It can be done, but you would have to be very careful about stocking it.
If you want to stay with SA (which is kinda my thing too, if you didn't notice LOL) I would have to suggest an Oscar. He'd have to be the only fish in the tank, or maybe some dithers at most, but they are really awesome fish.
If you'd rather have more than one fish, you should look into the medium-small, more peaceful cichlids like Acuras and even Rams or Apistos.
Another option would be to go with African cichlids, they are aggressive, but most stay small enough that, with enough territory, you can keep a good number in a 55gal. The africans (expecially Mbunas) seem to be quite popular for their beautiful coloring and great personalities. (although I still prefer SA cichlids
)