Bad Casting Choices in the Movies and Television

George Lazenby in On Her Majesty's Secret Service
The kicker on that is it's one of the very bet Bond scripts and otherwise has an excellent cast. If Connery had been in it, it would have had a chance of have eclipsing Goldfinger as the best Bond film. Instead, it's woefully short because of a terrible leading man.
And speaking of Bond, Pierce Brosnan. Roger Moore was always lame, but Brosnan was unwatchable.

Collin Farrell in Alexander the Great. Also in Daredevil, but that entire film was a miscasting disaster.