Bull sharks today.
Bull sharks are not the only saltwater species that can enter and thrive in freshwater, but they are easily the most intimidating. In fact, forty-two other species among the subclass Eslamobranchii, to which bull sharks belong, can live in freshwater. The way they manage this is through a process called osmoregulation, where the concentration of water in an organism's body is maintained despite changes in the external environment. All fish undergo osmoregulation; bull sharks are just particularly good at it. By controlling the concentration gradients of solutes in their bodies, bull sharks can ensure that the proper levels of water and solutes diffuse through them (by the way, this means that in freshwater bull sharks pee a lot).
I like a creature that knows how much salt it wants.
Here is an image of the geographic range of the bull shark:
A couple other things about bull sharks:
1) In Nicaragua bull sharks are known as "Nicaraguan Sharks," probably because Lake Nicaragua is one of their usual haunts. I, personally, think it's a load of bull (pun absolutely intended) that Nicaraguans would be so presumptuous.
2) The bull shark's diet consists mainly of bony fish and other sharks. (What is this world coming to?) They also eat a variety of other marine life (including dolphins, how sad), birds*, and terrestrial mammals (by which I'm assuming Wikipedia means people).
*I've always wondered about those birds chilling on the water amid violent bloodbaths on Animal Planet. I always assumed the sharks left them alone. But now I want to shout at the screen, "Fly, birds! Use your wings--they want to eat you too!"