If I had to decide, it would probably be Lee Grant as Leslie Williams. Partly because of how great she is in the role, and partly for "biased" reasons - she's the only one (at least in the original series) who works as a genuine "femme fatale" for me. Any given look she gives and any given line she says is just plain hot.

I go with Kay also. The actress played her very well. Considering her possible motives, none of which really gets us sympathetic (discussed on this forum), we can feel a wide range of emotions about her.

Another one for me is Nora Chandler. It's an easy expression to throw around, but Anne Baxter was a "Golden Age of Hollywood" actress, and she gave that kind of quality to Nora Chandler the actress.

These are big SPOILERS for anyone who hasn't seen it.

She manages to make you sympathize with her in spite of what you find out along the way about her murdering her husband, her deliberately (not accidentally) killing Jean, and her trying to kill Jerry. At least, to me it does.

I have to go with Jon Bird in this one. RIP Mrs. Columbo was under rated but was a great episode. Her character was portrayed perfectly.