Ok so I’m cheating slightly with this one. “Best of Both Worlds” is really two episodes, with the first part acting as the Season 3 finale, while the concluding part was the premiere of the following season. But nit-picking aside, the arc would go down as perhaps the most important in Star Trek history.
The two-parter saw the return of the mysterious cybernetic race known as the Borg, who capture Captain Picard and assimilate him as their spokesman. The ending to Part I is simply unforgettable TV - Riker, acting as ship captain and faced with the transformed Picard threatening his crew, gives Worf the order to “fire”. In an era where series regulars rarely got killed off, and especially for a show like Star Trek which rarely tampered with the status quo - this was shocking stuff.
Admittedly the second half doesn’t quite live up to the promise of the first, but “Best of Both Worlds” remains a turning point for The Next Generation. Prior to this episode, the show had mostly stuck to a tried-and-tested “monster of the week” format, but this arc had repercussions that were felt later in the series.
The very next episode of season 4 (titled “Family”) sees Picard struggle with the psychological scars of his encounter with the Borg, while this theme was explored further in the superb big screen outing First Contact, which pitted the Enterprise crew against the sinister cyborgs once more. Ultimately this is the episode that proved The Next Generation, which had spent its formative years playing it relatively safe, was prepared to take risks to tell its story, as it would do later with such classics as “The Inner Light” and another memorable two-parter, “Chain of Command”.