"King Boxer" (1971), retitled "5 Fingers of Death" when released in the USA in 1973. Lo Lieh (almost entirely devoid of charisma) stars as a young Kung Fu fighter who is learning the feared Iron Fist Technique. His teacher's rival sets out to destroy his school, cripples the Lo Lieh character to halt his progress, and imports Japanese mercenaries to wipe out all opposition.
The synopsis sounds crap, but it's magic. What really makes it is a secondary character, the street-fighter Chen Lung (played by Gam Hei Chu), whose hairstyle and charisma are both riveting. His rivalry with the hero, his employment by the villain, and his final redemption in the film's secondary plot arc, is what turns a run-of-the-mill Kung Fu film into a classic.
Well, that and the fact that it was the first Hong Kong Kung Fu film to be released in the West, and it created the market for Bruce Lee.