The animal that has has the most potent posion is a frog. They are called dart posion frogs and you can find them in Central and South America.
The poison is produced in glands and is secreted onto their skin, 2/10 of a microgram is lethal for humans if it gets into the bloodstream, so please, don't go licking frogs trying to find your prince.
However, that's only if you're considering vertebrates. The Box Jellyfish (which isn't a jellyfish by the way) has by far the most deadly - and also one of the most painful - poisons. It has been reported that it is so painful that victims continue to scream after losing consciousness.