I personnally like very much the one in the episode 43 "make me a perfect murder", played by Trish Van Devere.
She represents a very "human" killer, with a lot of feelings, who tries to play "though", especially in a man's world and profession especially at that time, but seems to have too many emotions, and be too compassionate to achieve her goals.
We can feel it even through the way the character is played, with a low voice, almost like a mother talking to her child.
Well it's the impression I had, anyway, and each time I watch this episode, I feel touched by this character, despite the cold-blood murder...