Paleoneurology
Today we are able to tell the intelligence of a dinosaur according to it EQ (Encephalization Quotient). The technique is to take the ratio of the brain's mass to the mass of the animal's body. In order to do this with dinosaurs, an endocast is done. An endocast is when a cast of the inside of the skull is made. Surprisingly, endocasts reveal pretty accurate descriptions and sizes of the dinosaurs' brains. With the EQ, we know that the dumbest dinosaurs included the Sauropods (Apatosaurus, Diplodocus, Brachiosaurus) and others, and the smartest dinosaurs included Dromaeosaurs (Velociraptor, Deinonychus) and Troodontids (Troodon)
It was also believed that dinosaurs like Stegosaurus had two brains. But research has shown that the second brain, which was located in the tail, was actually an enlargement of the spinal cord, and may have controlled the hips, back legs and tail. This enlargement in the spinal cord was larger than the creatures brain itself.