Who are you?
I am the Borg.
That is a contradiction. The Borg have a collective conciousness; there are no individuals.
I am the beginning; the end; the one who is many. I am the Borg.
— Data and the Borg Queen, First Contact
The Borg Queen made her debut in Star Trek: First Contact (1996), the eighth theatrical movie in the Star Trek franchise, and the second of the Next Generation movies. The character was sometimes played by Alice Krige and other times by Susanna Thompson. She made her first appearance as a disembodied upper torso and head, which was set into a robotic lower body, probably more for the nifty special effects visual than any practical reason.
According to all the on-screen dialogue, the Borg Queen was apparently intended to be a representation of the Borg's collective consciousness, not an individual in her own right, but rather an avatar to speak for the collective. Had this been the case, the Queen would have functioned in a similar manner to Locutus in The Best of Both Worlds. She would have presented an identifiable face and a personality for others to speak with. However, in practice this was not the case.
Despite the popularity of the Borg as villains, they made only a handful of appearances in the Star Trek franchise before First Contact. The same relentless, unstoppable nature that made them a believable threat to the Federation (indeed, to the galaxy) made them very difficult for the writers to handle. Obviously, the Borg couldn't win or there wouldn't be any more Star Trek. But how to defeat them? They were all but invulnerable to the Federation's conventional weapons, and there were too many to deal with individually. They were a triumph of massively redundant distributed design. They had no single point of failure. The answer seemed to be to give them a vulnerability. The Borg Queen was retconned into Star Trek continuity with a flashback to Locutus meeting the Queen after he was first transformed from Captain Picard.
To many Star Trek fans, the Borg Queen represents everything that ruined the Borg for Star Trek. As the trickster-god Q initially described them in Q Who?: "Interesting, isn't it? Not a he, not a she, not like anything you've ever seen before. An enhanced humanoid." The Borg had no need for gender, personality, or individuality. In contrast, the Borg Queen was unquestionably female, seductive, and not only an individual, but clearly a vital central figure in command of the Borg collective. Where the Borg were once unified, collective, interchangeable, disposable cyborg zombies, they now had a leadership figure — that is, they had a vulnerability. By the end of First Contact, it was apparent that destroying the Queen would deal the entire Borg collective a staggering blow, from which it would take some time for them to recuperate.
- Star Trek: First Contact (Alice Krige)
The Borg Queen invades the USS Enterprise 1701-E where her drones (sporting a new nanotechnology-based assimilation method) attempt to take over the ship. She seduces Data with the promise of incorporating organic parts into his android body (making him a sort of reverse cyborg) to get needed encryption codes from him. When she is killed, the Borg infestation of the ship comes to an abrupt halt.
- Dark Frontier (Susanna Thompson)
The Borg Queen attempts to bring Seven of Nine back into the collective to acquire all of the knowledge she has gained as a member of Voyager's crew. Perhaps the closest appearance of the Borg Queen to being handled appropriately, as an avatar of the collective to deal with outsiders on a personal level.
- Unimatrix Zero, Part I and Unimatrix Zero, Part II (Susanna Thompson)
This two-part episode doesn't even pretend to think that the Borg Queen is anything other than the absolute dictatorial ruler of an army of cyborg zombies. She has drones dissected with callous pragmatism and makes the decision to blow up an entire Borg Cube ship to slow the spread of a mutation in the Borg collective.
- Endgame (Alice Krige)
The final appearance (chronologically, not counting the future-drone One) of the Borg sees a future version of Kathryn Janeway infect the Borg Queen, with a "neurolytic pathogen" that disrupts her connection to the rest of the collective, bringing "chaos to order". The collective is last seen in uncoordinated disarray.