I think that Cahill U.S. Marshal (Andrew V. McLaglen, 1973) is a bit underrated. Many film fans don't like the sort of Disney quality that comes with a story focused on children, but the moral dilemmas are engaging and the theme of generational fracture is relevant and vital, for that time and for all times. I'm sure that many troubled fathers and sons can relate to it. It may not be a truly significant movie, and the filmmaking is probably mediocre at best, but it's worth viewing, and George Kennedy makes for a memorably menacing villain.