Darth Plagueis:No other installment of the series would be able to top the genious of the tandem that was George Lucas - Lawrence Kasdan - Irvin Kershner.
Actually, Leigh Brackett was responsible for most of the writing on Empire. She wrote virtually all the first draft of the screenplay before she died and Kasdan came in to work on it later because Brackett didn't have time to rewrite or polish. But if you read all the drafts, the great writing on Empire is largely down to her, not Kasdan. And remember what he and Lucas managed to do with Jedi's script...
You also forget Gary Kurtz, who produced both Star Wars and Empire, but not Jedi. He might have had differences with Lucas, but he did a hell of a job on both Star Wars movies - the best two - that he produced.