Hey, why not? I'm bored.
The answer to this one should be obvious: "The Empire Strikes Back." The first movie in the original trilogy was just fun popcorn sci-fi. The sequel brought compelling character drama into the story, and unlike the prequels, actually did it well (which makes me wonder why George Lucas didn't just stick with his role as executive producer for the prequels). Darth Vader got promoted from glorified enforcer (which, if you really watch "A New Hope," that's all he was) to an actual villain with a motive. And honestly, he's more threatening in this movie than he is in any other episode of the saga. Also, I maintain that the All-Terrain Armored Transports are a hundred-thousand times more awesome than any vehicle we saw in the prequels. That's right, George Lucas: stop-motion models on a sound stage covered in baking soda looked more convincing than CGI.
In short, the first movie was just entertainment. The second movie showed the world that the Star Wars universe had balls. It's only a pity that they couldn't keep that momentum going in "Return of the Jedi."