Sylvia Sidney
Though I'd say all the stars gave pretty good performances in this, I think Sylvia Sidney gave the best. She doesn't play it one note like some of the others do. She conveys a variety of emotions in just a handful of scenes and a few lines. Way more than was even necessary, really. Even though the film overall isn't great, I'd say she helped to make it as good as it could have been!
Maybe I appreciate her performance all the more just because this was her final movie, but even so I thought she was great.