Cronkite's four most well-known historical events during his tenure were the JFK assassination, the lunar landing, the Vietnam War, and Nixon's resignation. Of the four, I believe Cronkite's ominous conclusions about the Vietnam War were his most important contributions to America and his most sterling achievement as a journalist. Dan Rather called his review of the Vietnam War an "objective opinion", which seems contradictory but actually explains Cronkite's approach to the craft perfectly. Cronkite was a fierce and avid reporter, but he always reported the news. Rarely did Cronkite inject opinion into his reporting, but his most memorable moments were when he did exactly that.
Cronkite observed, reported, and concluded about the Vietnam War by actually going over to Vietnam. Here is an interview he gave a few years ago on what his thoughts were about his editorial on the war:
His opinions were based on what he saw, plain and simple. And everyone knew he meant it.
This is not to say, however, that Cronkite was best when he gave his opinions. His opinions would not have had much sway without the many years of matter-of-fact, honest reporting that made him a giant in the field. And his death yesterday is a reminder of how far journalism has evolved since his retirement, how much he shaped it, and how necessary it is for today's generation to look back and learn a few things.