The films he had the most impact on were the worst, sans the original eponymous classic, which is, IMO, the greatest of the series and perhaps the greatest film of all time...which was George Lucas's complete brainchild in nearly every aspect...I really can't believe he degenerated so much...
I wonder how he would have directed ESB if Kershner had not, or how the plot would have differed without the involvement of Brackett and Kasdan. I guess we will never know...