Q Who
Fun fact: Star Trek creator Gene Roddenberry originally planned for the Ferengi to be the main villains of TNG but, after a few episodes, the writers decided they were too comical. Hence the introduction of the definitely not comical Borg mid-way through season 2 in “Q Who,” which made it clear that these were the biggest threat we’d seen Starfleet face yet.
When Q appears on the Enterprise demanding to join the crew, Picard makes the mistake of saying they don’t need him as they can handle anything they come across. Q then transports the ship into a distant region of space where they encounter the Borg for the first time. Hopelessly out of their depth, Picard has to plea with Q to help them against the might of the Borg. It’s not quite as world-shaking as the iconic next appearance of the cyborg race, but “Q Who” is definitely a landmark episode in Trek lore.