There is no definitive answer to this question as it depends on a number of factors, including what you mean by “smartest.” If you are simply asking which species of big cat is the most intelligent, then the consensus seems to be that tigers are the smartest. This is based on a number of factors, including their ability to learn complex tasks, their use of tools, and their capacity for social intelligence.
However, if you are asking which individual big cat is the smartest, then the answer may be different. For example, there have been reports of individual lions and leopards who have been taught to perform complex tricks or tasks by their owners. In addition, some captive big cats have been observed using tools in order to obtain food or water (such as pushing a lever in order to open a door). So while tigers may be considered the smartest species of big cat overall, there may be individual lions or leopards who are just as intelligent â€“ or even more so.