Boston Celtics guard Marcus Smart is one of the best defensive players in the NBA, so he's uniquely qualified to speak on the debate over which player in the league is the most unstoppable. Smart was posed this question in a recent video done by The Players' Tribune, and his pick for the league's most unstoppable player wasn't Los Angeles Lakers forward LeBron James, Milwaukee Bucks forward Giannis Antetokounmpo or Brooklyn Nets forward Kevin Durant (when healthy, of course). It was Kawhi Leonard, who's in his first season with the Los Angeles Clippers after signing with them in free agency over the summer.