Casting was one of the show's great strengths, even into the third season, when the writing deteriorated a bit. A couple of standouts spring to mind . . .
"Gem" from "The Empath," (Kathryn Hays) a remarkably expressive performance, particularly given that it's done entirely in pantomime, without dialogue.
"Garth of Izar" from the above-mentioned "Whom Gods Destroy," (Steve Ihnat) a showy, scenery-chewing madman role, yet with an undercurrent of dignity, which more fully reveals itself after the character is "cured" at the end.
I believe I recently caught the tail-end of the Joan Collins episode. Kirk had to stop Bones saving her from an automobile accident at the logical insistence of Spock in order to prevent earth history from taking an alternative "wrong" turn. Puts a new spin on the expression, "how do you like your history."
