Locutus
Locutus of Borg was the Borg designation forced upon Captain Jean-Luc Picard after his assimilation in late 2366. The Borg intended to use Picard as an intermediary, a spokesman for the Human race in order to facilitate the assimilation of Earth in order to make the process as quick and efficient (or as perfect, if you keep the Borg's driving force in mind) as possible with the fewest number of casualties on both sides. This suggests that the Federation was one of the most advanced civilizations the Borg tried to conquer, certainly more advanced than they were used to and possibly even realised the Federation could pose a challenge. So they decided to try a different approach rather than their usual assimilation techniques of arriving and assimilating worlds one sector at a time.
Picard's assimilation also allowed the Borg to acquire the whole of Picard's knowledge and experience, as well as his own personal knowledge (a fact that was made apparent when Locutus addressed Commander Riker as "Number One"). Picard's detailed information regarding Federation technology and strategy yielded the Borg a significant tactical advantage when Starfleet confronted the Borg Cube at Wolf 359. (DS9: "Emissary")
This access proved two-way however, as the crew of the USS Enterprise-D was able to capture Locutus and use his link to disable and destroy the Borg vessel by sending the Borg cube a command to regenerate, creating a feedback loop that destroyed the cube and severed Picard's link to the Collective. (TNG: "The Best of Both Worlds", "The Best of Both Worlds, Part II")
Though his implants were removed and his wounds were allowed to heal, Picard's assimilation continued to haunt him. He returned to Earth, paying a visit to his family in La Barre, France, where he eventually confronted his demons. (TNG: "Family")
When Benjamin Sisko made contact with the Prophets in 2369, one took the form of Locutus as he appeared on the viewscreen of Sisko's ship, the USS Saratoga, shortly before the Battle of Wolf 359. (DS9: "Emissary") Locutus with the Borg Queen
When the Borg attempted a second invasion in 2373 (See: Battle of Sector 001), Picard's experience with the Borg allowed him to pinpoint a weakness in the Borg defenses, allowing the fleet to destroy the invading vessel. It was later revealed that Locutus was to have been more than a mere drone. The Borg Queen had intended for Picard to become her counterpart, an equal, a way to bridge the gap between humanity and the Borg. However, when Picard refused to give in to the Queen and accept his assimilation, despite being incapable of moving even a finger following assimilation, the Queen was forced to turn Locutus into just another drone. (Star Trek: First Contact) Even Borg Drones such as Third of Five still recognized Picard as Locutus after his separation from the Collective, possibly because the Borg's belief in their perfection meant that they couldn't believe anyone would actually leave the Collective (coupled with the fact that his memories presumably remained in the Borg's hive mind). (TNG: "I Borg", "Descent")
When Captain Kathryn Janeway was conducting negotiations with the Borg in 2374 during the Borg-Species 8472 War, she requested to speak to an individual, citing to the Borg that "You've done it before. When you transformed Jean-Luc Picard into Locutus." (VOY: "Scorpion, Part II")
