It's the episode that knits everything together. ANH was great, no denying. But Empire was the film (IMO) that truly made the story compelling. The Rebellion were up against a relentless and formidable Empire. The odds were heavily stacked against them, romance blossomed in the face of adversity, the hero's journey was fraught with danger and intrigue. The whole story was taken to another level. It's just a testament to fantastic story telling.