Whenever anyone thinks of a crossover episode in Star Trek, they probably think of Unification. It is biggest crossover during the course of The Next Generation and probably the entire franchise. The episode got massive ratings when it was broadcast in the United States; both halves of the episode were some of the biggest numbers that TNG ever received. The first half of Unification also saw the final appearance of Mark Lenard as Sarek, and we see the character's death during the course of the episode. Spock himself doesn't appear until about the final frame of that episode but does appear throughout the second one. It wasn't just the appearance of Leonard Nimoy that made this so great, but also Denise Crosby as Sela and the overall plot. The theme of unification has been brought up on several occasions following this and alluded to in the final season of Enterprise. Unification came during season five, often considered to be one of the high points of the series. This episode is the high point of that season, making it one of those episodes along with Best of Both Worlds and Yesterday's Enterprise that can truly aspire to be the best of the best.
