The worst casting decision I ever saw was Keanu Reeves in Much Ado About Nothing as Don John. He basically played the role as Ted "Theodore" Logan. And I really like Keanu Reeves. But he had no business in this role.
They also cast Denzel Washington as Don Pedro in that movie. Which he actually pulled off surprisingly okay. Michael Keaton also played Dogberry in the movie. He basically did Beetlejuice, but it kind of worked.