Parasitoid Wasp vs. Spider
Unlike the other animals on this list, spiders take a more modest approach to human inconvenience: they generally keep to corners, they stay quiet and they don't gnaw through trash bags in swarms. Spiders, in fact, are nearly invisible, and that's exactly the problem. Archnids across the board are surprise irritations, they show up when you least expect them, crawling across your pillow in the night or building expansive, imperceptible webs in walkways, always exactly at head level. Additionally, some of them are extremely poisonous. So, while I am vocally appreciate of their willingness to kill insects and hang the corpses up on display, I despise them in silence.
God damnit, you spiders.
Enter the Hymenoepimecis argyraphaga, a parasitic wasp that kills spiders for a purpose. In the hierarchy of awesome animals I created in third grade, wasp and spider weren't that far from one another but I'm willing to align myself with this particular wasp because it's so unrepentant in the way ruins the life of the Plesiometa spider. The wasp will sting the spider, paralyzing it just long enough to lay a single egg on the spider's abdomen. The egg will hatch and the larva will gradually suck the blood out of the spider while living on its back for two weeks.
Look at that stupid spider.
Then, when the wasp is developed enough to pupate, it releases a chemical into the spider which forces it to weave a cocoon for the wasp, a task the spider has never done in its life yet somehow accomplishes with tremendous skill. As a thank you for all the hard work, the wasp larva injects a poison into the spider and sucks out the remaining fluids before curling up in the cocoon and taking a nap. Ha. Yes. Take that, spiders.
Wolbachia vs. Cockroaches, Locusts, Wasps , Mites, Flatworms...just everything.
Imagine for a moment a world in which men became obsolete, where every birth was an immaculate conception and the only babies born were girls. Additionally in this world, the few men who are around start turning into women or dying of a deadly disease. The prospect is horrifying.
All those unopened jars, just sitting there.
Now imagine it happening to cockroaches and join me in a long, hearty laugh at their collective misfortune. The Wolbachia is a parasitic bacteria that attacks cockroaches, wasps, moths and even other parasites, turning all the males into females and ruining all sexual encounters. Unlike most other parasites, the Wolbachia infects the sex organs of arthropods in two primary ways: 1) It causes males to develop into females 2) All the females, including the transsexuals, start producing offspring asexually. What's more, the offspring are all guaranteed to be female as well. The reason for all this emasculation and forced celibacy is that the parasite can successfully infect the next generation from the point of conception by passing through the arthropod's eggs, but it can only achieve it through females. As a result, it's cut males out of the equation entirely. The bacteria will even kill any males during larval development or force them to develop as females, and destroy the immune systems of infected adult males so they die off.
The Wolbachia is the number one parasite on this list because it doesn't just kill a few hosts, it destroys the sexual mechanics of entire species, forever. And it's not isolated to one continent or habitat either, the parasite can infect around 60% of arthropod species around the world, ruining the lives of shitty insects, flat worms and spiders everywhere, and just generally playing God. I would offer the Wolbachia a figurative handshake if I wasn't so terrified of what it would do once it knew I existed.
You win, thing.