With all respect to bravozulu, I would go with anaconda. It can and in fact has, swallowed caimans, although a caiman is the largest creature an anaconda can swallow. The anaconda has the greatest girth of any known snake, up to the width of a telephone pole. It's surprisingly fast, aggressive and agile for its size. Its strength can crush a human. The bull shark is one I would describe as second "king" even though unlike the anaconda, it can't attack on land.
I would say that it is probably the black caiman.
"The black caiman is the largest predator in the Amazon ecosystem, preying on a variety of fish, reptiles, birds and mammals.[3] They are generalists and an apex predator, potentially capable of taking any animal within its range, including other predators.["
http://en.wikipedia.org/wiki/Black_caima...
Other candidates: Army ants, jaguar, anaconda, bull shark, pirrana....
And Why?
