The show's most memorably chilling villains, the Borg, are introduced in this riveting, genuinely scary episode which also sees the return of Q.
Few episodes of TNG managed to make space travel seem as alien, momentous and downright scary as this. Most of the time the Enterprise was just a travelling hotel in space and most of the aliens and races encountered were rather blah. But this episode stands out as a stark reminder of how scary the unknown can be and is an all-too rare example of when the Enterprise really DOES go where no one has gone before.
It's well-written and expertly produced, with a viscerally claustrophobic and intense tone throughout. The directing, music and production values are all first rate. Q's characterisation has rarely been better: funny, acerbic, but also dangerous and unpredictable - and I loved his rivalry with Guinan, which was an inspired touch. The Borg themselves are a brilliant creation: cyborg zombies that really did manage to strike fear into my heart as an impressionable young kid. I swear I had nightmares after watching this episode.
Overall, this is a smash and deeply invigorated an otherwise lacklustre season. Brilliant.