Well in reality since Humans are Animals then Humans would be the most smartest species of animal in the world.
But if you're asking about other animals other than humans then the smartest animals in the world are the Chimpanzee, Goats, Dolphins and Elephants.
It's been shown that in some memory tests done on humans and the Chimpanzee that the Chimpanzee actually did better at the memory test than humans.