Three's a Crowd
Very good episode from the classic second season and one of the show's most mature. Different from the standard Horror fare in that it's more psychological and emotional, concerning a jealous husband with a drinking problem who suspects his wife and the family friend are cheating and he's pushed to take murderous actions about it. Notable for it's shocking ending that stays with you long after you've seen it. Just thinking about the ending can put me in a glum mood.
Very good episode and one I rate highly, even if it's more depressing than your usual TFTC fare.