Sure, there are sci-fi films better written, or with, now much more grander & updated special effects. But other than Star Trek, none have the expansive imagination or have been able to touch the heart of an entire generation of nerds, particularly with such a long lasting impact, as the original Star Wars trilogy.
And while I know that Star Trek has affected more numbers of sci-fi geeks, but, for me, as "big" as their stories & concepts were, they never seemed to go all out with their depictions of aliens races as much as Wars. And as integalactic as they tried to make their stories, I never felt that they were as epic.
For the movie of Empire itself, I agree with the rest of the world that this is the pinnacle of the entire Star Wars franchise, with it's darker tone & cliff-hanger ending within it's still fairytale-like structure. The saga of the intergalactic war between the Empire & the Jedi-led Rebellion hits it stride as Luke Skywalker, Hans Solo, Princess Leia & the rest of the crew find themselves pursued & cornered by Darth Vader & the forces of the Dark Side.
The Empire Strikes Back came out at a time back when movies would stay in wide release, not for weeks at a time, but for months, and a few of 'em, like this one, for almost a full year.
From my own point of view, I think that, on the majority, audiences today have been so desensitized from so-so blockbusters being released every other week-end or so, that they will never get to fully understand or experience the feeling a big movie-event like Star Wars. The year of Empire was one of my fave summers ever as a kid. It was really cool knowing that I could hop on the city bus at any point during the season & go watch TESB. And every time that I did, the other audience members were just as excited to be seeing this film at the end of the summer as they were at the beginning. Every year that one of the 3 original films was released, you could feel the power of the "Force" within the air, and not just until the next "big" movie came out. You could actually feel it in the air for almost the entire year long. And then some.