The dinosaur often considered the most intelligent relative to its body size is Troodon.
Troodon was a small, bird-like theropod from the Late Cretaceous. Its reputation comes mainly from its relatively large braincase, large forward-facing eyes, and other features associated with keen vision and active behavior. When palaeontologists compare estimated brain size with body size, troodontids rank among the highest of known non-avian dinosaurs.
That does not mean Troodon’s intelligence can be measured as precisely as a living animal’s. Brain-to-body ratio, or encephalization quotient, is only an indirect proxy; it cannot reveal language, problem-solving ability, or social behavior from fossils alone. Earlier reconstructions also inspired speculative ideas about a highly intelligent, almost human-like dinosaur, but those portrayals went far beyond the evidence.
The name itself is also a source of modern debate. Much of the brain and skeleton material historically assigned to Troodon has been placed in other troodontid genera by some researchers, while other researchers continue using Troodon for those fossils. In ordinary trivia usage, however, Troodon remains the standard answer.