My holding The Empire Strikes Back in such high regard could be as simple as this: Of all these movies, which is the only one to have Imperial Walkers, The Godfather, Shawshank Redemption, Pulp Fiction, Gone With the Wind or The Empire Strikes Back? See, I can almost make it that simple, but that's a just a tad short-sighted so I won't.
This reason is not as stupid as it sounds though. What makes fantasy and science fiction movies different is their ability to make your imagination go beyond your wildest dreams. In the Star Wars movies, the first couple anyway, George Lucas has the perfect blend of humans interacting with machines that as children we must own. Of course you need an Imperial Walker when you're a kid. The Death Star! The Millenium Falcon! That thing the Jawa's drive! Boba Fett's vehicle complete with frozen Han Solo! Naturally, other classic movies don't gear themselves towards a big sale at Toys R Us, but as a child seeing this on the big screen, I wasn't concerned about that the toys, I was concerned about the action and wanted the toys afterwards.
Now that I've seen this movie compared to other movies of it's time as a grown up, it's still all about the movie and not some targeting of children sideshow. Nearly every scene is classic, from the Tantans, to Yoda, to the asteroid belt, to the city in the clouds to the freezing of Han Solo. Sure the acting may be a sub par, but what you lack in acting is definitely made up for with everything else. My guess is it's pretty hard to have Imperial Walkers and Oscar winning actors in the same movie.
Speaking of actors in a movie, something else I've noticed that is yet another difference as to why the first three Star Wars movies are so much better than the newer ones, is people. The original trilogy, despite a few odd characters here and there, is an Empire of men versus a Rebellion of men. There's no CGI of characters with speech impediments no matter how hard Lucas tries to ruin the originals. If Lucas could've kept the prequels more human, he may have had better results. For instance, maybe more humans and Hayden Christiansen would've known how to act. I wince at the thought of Mark Hamil having to interact with CGI characters after the job he does talking to the minions of Jabba's Palace.
Maybe the reason why George Lucas went with Hayden Christiansen is because he figured, "Shit, if Darth Vader is such a good actor in the prequels, how is it going to look when his son Luke is a terrible actor?" That's the only explanation that makes sense, well, that or Lucas is a complete idiot and doesn't understand how to make movies that appeal to anyone with a 5th Grade Intelligence Level or higher.