Star Trek: Voyager's Kathryn Janeway (Kate Mulgrew) deserves the honor of being thought of as the Borg's greatest enemy, a distinction that is often given to Jean-Luc Picard (Patrick Stewart). Janeway was the captain on Voyager for all seven seasons, holding the distinction of being the first female captain to lead a Star Trek TV show. Despite the Borg being most closely associated with Star Trek: The Next Generation, seasons 4-7 of Voyager also heavily featured them as the main antagonist, with the crew of the USS Voyager coming into conflict with the Collective and the Borg Queen on multiple occasions during their journey through the Delta Quadrant.
Both Captain Picard and Captain Janeway had their fair share of dealings with the Borg, and there is good reasoning for both to be considered the Borg's greatest enemy. However, in Star Trek: Voyager's series finale, "Endgame", it was Janeway who dealt the Collective the ultimate blow in the form of a neurolytic pathogen. A time-traveling Admiral Janeway allowed the Borg Queen (Alice Krige) to assimilate her and subsequently infect the entire Collective with the pathogen. The effects of this attack weren't explored fully until the series finale of Star Trek: Picard, which finished what Voyager had started and finally killed the ruined Queen and her Collective once and for all.
