Disagree. While Empire might be the best Star Wars film, in my opinion the best FILM of the saga is Star Wars. That is, Empire is arguably the most defining and impactful piece in building the saga (the force, the family, the Yoda, the love story, the redemption, etc). But if each film were to be judged against the landscape of great films, in my opinion the only one that really stands on its own at a comparable level alongside the greats like Citizen Kane, Casablanca, etc., is Star Wars.
The quality of the making of the film Empire Strikes Back is, for the most part, superior to the quality of the making of the film Star Wars. The script is better. The acting is better. The characters are more three dimensional. The framing and blocking is better. The cinematography is better. The score is better. The pacing of the action is better. The special effects are better.
As good a film as Star Wars is … and its very very good … the only filmmaking aspect that stands up well to the quality of Empire is the editing, in my opinion.