I will probably catch a lot of flak for this- but my nominee for really bad acting in a great movie:

Vivian Leigh in Gone with the Wind .

Hysterical, over the top (and not in a good way) and just … awful. I never bought that she was a schemer, or cunning . Her take on the character comes off as one dimensional. I have not read the book , so maybe Scarlet was written that way.

Do I like the movie? You bet. But that performance always grates on me.