What are the dimensions of the tank out of interest? - If you're looking for a single larger fish then width an important factor.
Do you want a single fish or a 'community' of aggressive fish, I would think a single larger 'wet pet' would probably be best due to limited tank size. My suggestion for NW cichlids would be a female midas/red devil. A texas cichlid would also be a contender. Possibly a female trimac if you can find one (most will be flowerhorns or crosses).
If you wanted something smaller then theres fewer nastier fish than salvini's, you could have a pair of them in that size tank. Equally convicts are probably pound for pound one of the most aggressive cichlids, though I'm not sure if a pair would suit your taste, maybe a salvini plus a convict?
As for oddballs then my ideas would be from rainbow wolffish / dwarf snakeheads / exodons / however the most aggressive oddball I can think of just while typing now is probably a congo puffer.