I would have to vote for the Oscar and the labidochromis caeruleus. My grandmother had 2 oscars. One poked the eye out of the other oscar. And when my daughter goes up to the tank one of the oscar attacks the glass where my daughter is. It's as if it wants to eat her face off. As for my labidochromis caeruleus, they've killed a betta I had, some guppies I had, they try and eat the dorsal fins off of my moonlight gourami, they are always fighting with my pleco, and they all alway fighting with each other. I also have some long tank tongs that I use when I need to put a plant or new decoration in. Well, when I put it in the tank the labidochromis start attacking it.