There is no definitive answer to this question as it largely depends on the definition of "largest brain". For example, if measuring by sheer mass, the whale shark has the largest brain of any fish species. However, if measuring by proportions or relative size to body, then the mola mola (ocean sunfish) is generally considered to have the largest brain of any fish.
When it comes to intelligence, however, there is still much debate as to which fish species is truly the smartest. Some scientists believe that certain members of the ray family are among the most intelligent fish due to their complex social behaviors and ability to use tools. Other researchers believe that fishes in the cephalopod family (such as squid and octopuses) are likely candidates for being the smartest due to their high levels of learning and problem-solving abilities. Ultimately, more research is needed in order to determine which fish has truly earned the title of "largest brain".