“The City on the Edge of Forever”
That one's part of Trek Trivia 101, Rob. Tsk tsk etc. “The City on the Edge of Forever” seems to be on everybody's list of Best Trek:TOS episodes Ever (it's on mine), I think for two basic reasons: An excellent story (by Harlan Ellison) and scriptwriting; and Joan Collins's strength as an actress, so that her character was strong enough to not be overwhelmed by Shatner's histrionics and general carpet-chewing. "Edith Keeler" humbled Kirk, in a way that few other females, human or otherwise, ever did.