To me, Empire is undoubtedly the best film of the trilogy.
The original Star Wars was a great film that, together with Jaws (as kb mentioned), helped usher in the age of the blockbuster. It brought ground breaking special effects, a new visionary universe, and some of the most iconic scenes and music to ever hit the big screen.
But as steve and others mentioned in the last thread, it was plagued by cheesy dialogue, hammy acting, a basic, thin plot, and, in my opinion at least, a lack of ambition with the camera. Star Wars undoubtedly had a huge influence on cinema, but so did Seven Samurai, Citizen Kane, 2001, Lawrence of Arabia, 8 1/2, and The Godfather (as well as many others), which all preceded it.
Then comes Empire.
The first aspect of Empire that sets it apart from the original is the huge difference in budget and freedom. The production values for Empire dwarf those of Star Wars. The sets are more detailed, the costumes are more modern, and the basic picture quality is vastly improved from the original. Empire makes the Star Wars universe feel more real and lived-in than the original, and that is largely because the production.
The acting is far better in Empire, too. Luke is still whiny, but it's for narrative purposes, and his whiny scenes come off less hammy in Empire then the chuckle-worthy dialog in IV ("But I was supposed to go to Toshi Station to pick up some power converters!"). Fischer also steps it up in Empire, making Leia less shrill while still retaining her strength and leadership. I also felt that Jones' voice work with Vader was improved, and that the voice mirrored the action in scenes better.
The camerawork also stands out in Empire compared to the original. I said in the last thread that the original didn't have very good shots, and while I stick by that for the most part, I will concede that when I watched and paid more attention to it last weekend, the film did have many good shots, but only in the first act and the last with the attack on the Death Star. I will say the scenes in the Death Star interior didn't use the camera very well.
Empire, however, still excels.
There are few scenes in the original shot as well as the Wampa cave scene or the Carbon Freezing Chamber. That's not even getting into the skillful work done to capture the Asteroid Field Scene or Luke vs. Vader.
Visually, Empire is a step above IV, and that really helps sell the world and ensure the saga would be respected for years to come.
However, my favorite aspect of Empire is its story.
Empire's plot is different from most sequels nowadays. It scales it back. It focuses less on a galactic conflict and more on the personal conflicts that tie these characters together. This helps make Empire a much more focused, yet deeper experience than the other films in my eyes (though I also love 2/3 of Jedi).
They were able to do this by basing the entire plot of this film around a single character: Darth Vader.
Empire, more than any of the others, is Darth Vader's film.
Star Wars introduced him as an iconic villain, but he was always leashed by Tarkin and never had much personal interaction with the main protagonists other than Kenobi. Yet, we knew there was more to him to explore.
This film made Vader the terrifying force he's remembered for.
Vader is on a mission throughout Empire as made apparent from the very beginning with the title crawl. He's out to get Luke Skywalker, and he will kill enemy or ally to accomplish his goal. The way he disposes of his admirals, the way he easily handles Han on Bespin, the way he shows no care toward Han or Leia, seeing them only as tools to trap Luke, the way he apathetically tests the carbon freeze on Han with no regard for his life, and the way he toys with Luke during their duel all serve to show how committed Vader is to capturing Luke, how ruthless he is in his methods, and how powerful of a force he is to be reckoned with. And while these scenes enhance Vader as a villian, they also give Vader so much more depth when you view them knowing the twist. He's ruthless and committed, but he's doing so in pursuit of his son, his only link to his humanity after he gave up on being a "good man" years ago.
Plus there's that dialog. Vader (and Yoda) will always have the best lines in the trilogy to me.
"You have failed me for the last time, Admiral."
"I am altering the deal. Pray I don't alter it any further."
Chilling lines that cement Vader as a great, terrifying Villain.
"The force is with you, young Skywalker, but you are not a Jedi yet."
"All too easy."
"Impressive. Most impressive."
Great commentary during the duel that expresses Vader's overwhelming power compared to Luke and also expresses his respect for Luke (and a bit of pride he has for his son).
These lines are not Shakespearean, but the way they are delivered, and the scenes they are delivered in help really flesh out Vader's character. His comments during the lightsaber duel make the fight more compelling and personal. It adds emotional weight to their duel, weight that was unfortunately missing from the myriad fights in the prequels.
Then there's that great twist that will forever live in cinematic history, a twist which makes everything bigger yet also smaller and more personal in this universe. ("No, I am your father.") When Vader later says "Bring my shuttle", you can hear the pain and disappointment in his voice as he thinks he has again lost his only son.
His humanity finally comes out at the very end when he is pleading with Luke to join him. Pleading. I mean that moment when the Falcon hits lightspeed, and Vader looks away from the Window, back again for a second, and then finally turns away? What an emotional scene! And only told through head movements!
By making the central conflict revolve around Vader, we're able to get a closer look at our other main characters.
Luke's whinyness and impatience now have a real narrative purpose, showcasing how tempting the Dark Side is and how green Luke is. The twist reaffirms this and also makes this conflict mean so much more for him. He's facing the former Jedi he thought was dead, his father, and his place in the universe has been elevated to an importance he hadn't realized (even after destroying the Death Star).
Han develops a protective relationship toward Luke (look at that look he gives Luke before the battle of Hoth, look at how quickly he gets angered when Lando tells them Vader is trying to trap Luke, and just look at the very beginning when he saves Luke's life). He's also fleshed out as the charmer and lover, through his pursuit of Leia. We seen Han as a more respected member of the Rebel Alliance in this film, and the calm acceptance of his fate is the noblest Han ever is.
Leia becomes warmer through her relationship with Han and the care she expresses for her friends. And her leadership shines in scenes like the the prelude to the Battle for Hoth, the escape from Bespin, and her decision to take command and get Luke.
Empire also adds more characters to the Universe that help flesh it out and make it feel larger, even while the plot has a smaller focus.
Yoda is one of the best characters ever. His explanations of the force capture your imagination, and his change from a slapstick puppet to a Jedi master is such a masterful surprise. And that dialog!
"Wars not make one great!"
"That is why you fail."
Seriously. Props to Frank Oz and the team that made Yoda so compelling and interesting. He really comes off as a wise master of ancient magic.
Boba Fett also inspired many fans, though his promise was short lived. We finally get to meet the Emperor for once in Empire, and Lando really rounds out the Rebel cast.
The music is also just at good as in the original. The Asteroid Field song, the Imperial March, Leia's Theme, and Yoda's theme are as iconic in Star Wars as the opening theme and the dual sun theme. And the special effects are fantastic with the Battle for Hoth being one of the most iconic moments of the entire saga.
I've already typed too much and not even covered all I could about what makes this one work so well, but there is just so much to this film that makes it great to my eyes.
I'll just leave with a list of the iconic moments that, IMO, make the film the best of the saga.
The Wampa Cave
The Battle of Hoth
The Asteroid Field
The Cave of Dagobah
Yoda Lifting The Ship
The Carbon Freezing Chamber
Luke vs. Vader
Oh and the great dialog that runs through the film (not just for Vader). Yoda in particular has some of the best lines in the trilogy. And there's of course the many great lines delivered by Han in this ("Never tell me the odds!" or "I know.")
Bah. This is the best. It may not be the happiest, but it makes the world so much bigger while making Luke's story so much more personal. It makes Vader a true central character, and sets up one of the best philosophical, emotional, and personal battles in cinema with the fight for his/Luke's souls.
PS: I forgot to add this, but seriously. Watch Chewy in the infamous kissing scene with Luke and Leia and then imagine what he's saying since he knows their history! I kinda cracked myself up with that.