The top prize for The World Most Venomous Animal,would go to the Box Jellyfish. It has caused at least 5,567 recorded deaths since 1954. Their venom is among the most deadly in the world. Its toxins attack the heart, nervous system, and skin cells. And the worst part of it is that jelly box venom is so overpoweringly painful, that human victims go in shock, drown or die of heart failure before even reaching shore. Survivors experience pain weeks after the contact with box jellies.
You have virtually no chance to survive the venomous sting, unless treated immediately. After a sting, vinegar should be applied for a minimum of 30 seconds. Vinegar has acetic acid, which disables the box jellys nematocysts that have not yet discharged into the bloodstream (though it will not alleviate the pain). Wearing panty hose while swimming is also a good prevention measure since it can prevent jellies from being able to harm your legs.
Jelly box can be found in the waters around Asia and Australia.