I personally think the defining sequence of the saga is the sequence of events from the revelation to Luke that Vader is his father to Luke deciding to fall down the shaft. I have never believed the revelation was meant to be a surprise to the audience. I think when Lucas made that move he meant for that to be the defining sequence in the saga. At that moment, the entire galaxy hung in the balance. When Luke fell instead of joining Vader, that set the stage for the end of the Empire and Vader. We've been told the saga of Star Wars is about the rise, fall, and redemption of Anakin. If that's true, I think the redemption is the climax of the story (Anakin killing Palpatine). I see Luke learning the truth and choosing to fall instead of joining Vader, as the set up point for the redemption and the defining moment of the saga. A lot was riding on that decision by Luke.