The animal with the biggest brain is the sperm whale. The average brain weight of an adult male sperm whale is 7.8 kilograms (17 pounds), which is more than five times heavier than the human brain. The sperm whale's brain is also very large relative to its body size, with a brain-to-body mass ratio of 1:5,100. This means that the sperm whale has one of the most complex brains in the animal kingdom.

The sperm whale's large brain is thought to be necessary for its complex social behavior, echolocation skills, and hunting abilities. Sperm whales are highly social animals that live in pods of up to 50 individuals. They use echolocation to navigate and hunt for food in the dark depths of the ocean. Sperm whales are also apex predators, meaning that they are at the top of the food chain. They prey on large squid and fish, which require a great deal of intelligence to hunt.

While the sperm whale has the biggest brain of any animal, it is not necessarily the smartest. Intelligence is a complex trait that is difficult to measure, and it is not always correlated with brain size. For example, the African elephant has a brain that is only about three times smaller than the sperm whale's brain, but it is thought to be more intelligent. This is likely due to the elephant's complex social structure and its ability to use tools.

Ultimately, the question of which animal has the biggest brain is a matter of definition. If we are talking about absolute brain size, then the sperm whale is the clear winner. However, if we are talking about brain size relative to body size, then there are other animals that are more intelligent.