John Sturges' HOUR OF THE GUN (Mirisch / MGM, 1967) obviously.
One of the best westerns of the 1960s. It begins with the gunfight at the O.K. Corral and then deals with the aftermath. It's about the attempt to kill the Earp brothers and Wyatt's vengeance ride. It uses more history than any other Earp film even though it gets the history wrong, it conveys a sense of gravitas. Dramatically it's a soulful bounding leap above and beyond Sturges' earlier classic GUNFIGHT AT THE O.K. CORRAL (1957). This isn't a sequel so much as a revision and re-imaging. A very fine film in any case with an atypically humorless and steely-eyed James Garner as Wyatt Earp and scene-stealing Jason Robards exploring the depths of cynicism as the tuberculor Doc Holliday.
A great score by Jerry Goldsmith, and great performances by James Garner (one of his best against type) and Jason Robards Jr.
I'd also add Steve Ihnat to that great performances list. He plays his last scene with Garner brilliantly, as his character is a weaselly sack of excrement right up 'til scene's end. Ihnat's delivery of his lines is nothing short of superb.