A film is only as good as its villain.
There have been five "Terminator" films released since 1984, with the newest installment of the franchise "Dark Fate" set for release in the fall. But, unlike other popular movie franchises, there has not been much emphasis on Terminator villains aside from the original Terminator played by Arnold Schwarzenegger in the film of the same name.
But 1991's "Terminator 2: Judgment Day" introduced the world to the T-1000 played by Robert Patrick, an advanced Terminator model made of liquid metal that can transform into almost anything it comes in contact with. "Terminator 3's" villain, the T-X, played by Kristina Loken, was fine and definitely held its own against Arnold's T-800, but she lacked the threatening nature and imagination of her predecessor.
"Terminator Salvation" was not based around one central villain but the Skynet corporation (the creators of the Terminators) as a whole, so there wasn't any real time for one core villain to get a focus.
Director James Cameron purposely wanted the T-1000 to be much different from what came before. This Terminator was a much sleeker, less bulky version compared to the original T-800 to give the latter more of an edge in combat and stealth. Unlike Arnold passing off as human but still lacking normal communication skills in the previous film, the T-1000 had those human interactions down and used it to his advantage when needed.
At first glance, he is a law enforcement officer trying to kill a boy and his mother. Arguably, for most people, police are supposed to represent safety and order, which further symbolizes the machine using the human shell to its advantage.
When there is a villain that can do anything, it creates an obstacle for the writer who has to come up with a weakness that does not feel convoluted or cheap. At certain points in the film, the T-1000 is even seen performing feats beyond the average robot (due to its liquid metal aesthetic) from melding with a hospital floor to transforming into another human being. It is not until the end of the film where this killing machine first displays its first bit of human emotions of fear and panic, moments before melting in the molten steel.
Throughout the film, the T-1000 remains a force of nature that can be slowed down and defeated temporarily but not killed. The villain does not have much in the way of personal story, but it isn't needed. Not every movie villain needs a major origin or backstory; the most effective are the ones without a story behind them and instead just one motive: to kill.
Alfred in "The Dark Knight" said it best, "...some men aren't looking for anything logical, like money. They can't be bought, bullied, reasoned, or negotiated with."
Robert Patrick's portrayal of the T-1000 proved so successful that the series ended up bringing the character back three films later with 2015's "Terminator Genisys", now played by actor Lee Byung-hun. Outside of the film itself, the T-1000 may not have ended up killing John Connor but still managed to take home several visual effects awards for the film. The visual effects done on the T-1000 by Stan Winston remain some of the best visuals done on any film and helped to give the character a life of its own before the molten steel took him in.
The T-1000 is the ultimate Terminator.