For fans of the show, this might be a controversial statement, given the justified popularity of "Marge vs. The Monorail," "Last Exit to Springfield" and "Homer's Enemy," but do hear me out. While "Lisa's Rival" may not be as hilarious as the "Monorail" episode or as innovative as "22 Short Films About Springfield," it best encapsulates what made The Simpsons so great in its prime--said prime lasting a roughly seven year period from season 2 through season 8 (just less than a third of the show's actual run, you might be interested to know).
