I'd have to put my vote for the BEST animal in the entire world as the Army-Ant's: Evidence by Cracked.com and many others if you bother to look the information up.
"Army ants are also, happily, completely carnivorous, and with millions of them in one place at a time, they essentially never stop killing, tearing through hundreds or thousands of prey an hour. They only stop killing for a few weeks a year, but don't relax -- that's just because they take a breather to make more army ants.
They're clever, too, moving by day in a variety of formations to protect the more valuable members of the colony, and to drive and trap their prey before them. Witnesses have observed army ant waves 65 feet wide. And at night, the ants stop, forming a colony out of their own bodies, full of tunnels, passageways, and public spaces, filled with what still must surely be a tremendous amount of murdering other species.
They can turn cow corpses into skeletons, and yes, have already been known to kill humans, although for now it's just been the slowest and weakest -- people unable to move out of the way of the relatively slow-moving columns. But don't take too much solace in that, because they can move underground, too. And were they to ever organize on a scale larger than a few million, that could change. Because boy howdy, there are a lot of them. In terms of total body mass, all the ants on Earth may outweigh all the humans, and when you consider just how enormous humans are getting, that amounts to a whole lot of ants." -Cracked.com
TL;DR = Army ant's will can eat entire humans and cows. They also come in swarms of up to millions and might possibly outweigh all the humans on earth according to bodymass.
TL;DR^2 = They will eat you, there are millions of them.
