I went with Luke vs Vader in The Empire Strikes Back. This culminates in the climactic scene that defines the whole series: Darth Vader is Luke's father. We had been laboring under the impression that Star Wars was the story of the rebellion's fight with the Empire, when in fact it goes to a deeper, more personal level of father vs son. After that revelation, the whole series became about Anakin Skywalker's fall to the dark side, and his redemption by his son, all of which wouldn't have been possible without that one scene in the cloud city. Plus, I can still vividly remember the audience reaction to what Vader told Luke, after all these years.
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-Bill, otherwise known as Kraichgauer