Oh goodness, I just watched Conscience of the King. Love the episode, but some of the dialogue from Kodos' daughter was laugh-out-loud awful - especially when she starts talking about throbbing...
But her end of show breakdown, mournful sobbing turning into maniacal laughter, is one of my favorite scenes.
Barbara Crampton sort of redoes it in Stuart Gordon's wonderfully over the top From Beyond
(starring Jeffrey Combs).