Kevin Costner is a very good actor and director but I think he was definitely not the right choice for the role of Robin Hood.
Neither was Russell Crowe.
Benedict Cumberbach as Khan. I just didn't get it.
I was a big 2000AD fan, worse ever casting for was Stallone as Judge Dredd.
Stallone wasn't badly cast as he had the jaw for the part. It was the removal of the helmet and the decision to lean into comic relief that pulled the movie down. (that said, it's a guilty pleasure and i have a little soft spot for it. )
Adam Sandler - pick a movie, any movie.