"Etude in Black" - Mrs. Benedict. In my opinion, she wasn't all there. She didn't have a very good self-esteem when it came to her husband, and herself. An example is when her mother ordered for her when she was in the restaurant. When she asked Alex why he married her.
Anyone who has a problem with "New Age" type people (I don't) might find Lisa Chambers at least a little unstable just because of that.
Arlene Martel causes me to have such a thing for Gloria West that I can't be very objective about her. Though she does seem pretty vindictive toward Cathy (even if she does believe she's guilty of the murder).