What's the Most Intelligent Animal on the Planet, Other Than Us?

Scientists ask this question all the time. For a long time it was assumed to be our closest living relative, Pan troglodytus, more commonly known as the chimpanzee. (In fact, our closest living relative is Pan paniscus, the bonobo or pygmy chimpanzee).
But with a lot of study, we've come up with some other candidates. Let's take a look.
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