Fingolfin High-King of the Noldor said:
Mosquitos. Anything that can eat a mosquito can eat any other insect and they just bother us humans.
Mosquitoes put the proteins humans develop back into the food chain by being consumed by frogs and fish. If you really want to give back to the biosphere, go for a nice hike and let the mosquitoes feast. Also, the mosquito's bite is anesthetized, so animals don't feel it, it just so happens most humans are allergic to the anesthesia. So it's our fault we get all itchy, not theirs. They're just trying to numb the sting. What would you rather have, an itchy bump or a sting? I still hate 'em.
Platypus aren't useless, that ridiculous bill lets them get at worms n such in the mud others animals can't find, they also have a highly developed electric sensory field to detect movement through a few inches of silt.
Koalas are pretty useless, but if they can survive in the poisonous hellhole of Australia, I think they deserve a vacation.
EDIT: Pandas are pulling a nice con on humanity, they're like the welfare cheats of the animal kingdom. I don't see them when I go to the zoo, and they're really just asshole animals. Some animals are friendly with their keepers, but not pandas.