^^ I think she was at her absolute loveliest in The City on The Edge of Forever - a two-parter from the very first Star Trek series, if you haven't seen it.
Harlan Ellison wrote the screenplay, which floored me because most of his stuff was extremely punk (as in rock, not as in lousy) at that time. And she was just incredible, and perfect for the role.
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"I believe you find life such a problem because you think there are the good people and the bad people," said the man. "You're wrong, of course. There are, always and only, the bad people, but some of them are on opposite sides."
-- Terry Pratchett, Guards! Guards!