I fall into what must be an extraordinary minority on this subject in choosing The Big Boss, which Lee himself was even said to have despised, as among my selections for favorite Bruce Lee films. I'm not sure I would call it my single favorite, but I think it's often unremembered and discredited for being more of a conventional Lo Wei production than the type of innovative martial arts film incorporating Lee's personal philosophic ethic like his later works would be, when really it showcases a wider range of Lee's emotive abilities than any of his other films, and also features the legend in what is probably the second fiercest, most brutal depiction of cinematic combat after Fist of Fury. The final tussle with Han Ying is an overlooked classic.
It also boasts the distinction of being Lee's first starring role, and consequently the movie that put him on the map with Asian audiences and ultimately enabled him to expand his acting and present his unique perspective on martial arts to worldwide audiences through later projects.