Seen as this is just a bit of hypothetical tomfoolery let me enter my badass entrant. Drum roll.....I give you.....my dwarf dovii. I'll stunt it's growth in a 1 gallon tank and then let it loose on your sorrowful choices. I win. I like this game, what categorie's next, lol.
I had to pick the puffer. Pound for pound meanest puffer and kills for fun. And besides, even if one fish wins and eats the puffer, TTX gets the last laugh.
I would say badis. Relatively peaceful...but not when other fish enter it's territory. Aggressive for such a small fish. Honestly, I would pick swordtails, because they are absolutely ruthless. But if i had to pick a different one, it would be badis.