I would have to say Vader's death/redemption. Before it was good - Luke/the Alliance won, Luke and Lei's dad turned good again, and it was sad - Luke and Leia's dad died. Now we can put a name to "Luke and Leia's dad," and there are more emotions attached to him. Now, Anakinhelped his son and the Alliance win, Anakin fullfilled the prophacy, Anakin finally stood up to Palpatine.....and then he dies! This isn't some guy anymore, it's somone we know and care about.