In anticipation of the upcoming sequel, here's a breakdown of the three act structure of the first Terminator movie, written by James Cameron and Gale Anne Hurd.
Main Character: This is a little bit tricky. On the one hand, Kyle Reese is introduced first, he has a strong goal, and he is the more physically active character through much of the movie. However it is really Sarah who has the biggest problem and ultimately it is her decisions that drive the story forward. Furthermore our hope and fear is centered around Sarah's survival. This makes her the main character.
Catalyst: When the Terminator kills the first Sarah Connor. This is the moment when the danger to our hero is crystalized for the audience, though Sarah doesn't yet know about it. This sets up...
The Main Tension: Will Sarah survive the Terminator?
Act I Break: When Sarah learns of the second killing and then spots Reese following her. She now realizes the danger she's in and begins looking for a way out - initially by seeking shelter in a night club and calling the police. (Very quickly she discovers that Reese is not the one she has to be afraid of.)
Midpoint: When Sarah and Reese escape the attack in the police station. This is a crucial moment because it is when Sarah finally decides to trust Reese. She went along with him earlier, but she didn't really have any other good options then. It is here where she becomes convinced he's telling the truth... and that he's the only one who can help her. This is demonstrated when she's hiding under the desk, hears his voice and identifies herself to him.
Act II Break: When the Terminator locates them at the hotel. Arguably you could point to the earlier moment when Sarah gives her "mother" her location because that is when the Terminator gets the information he needs to track them. But there is some time where the threat does not seem imminent - in fact we get the big love scene after this point. It still seems possible Sarah and Reese can avoid detection until the Terminator actually arrives.
Twist/Epiphany: This one is tough. It has to do with Sarah taking charge of the action. I would put it at the moment when Kyle collapses and she yells at him: "on your feet, soldier!" You could also make a strong case for the moment he dies.
Resolution: Sarah destroys the Terminator with the compactor. The main tension is resolved - our character has succeeded.
So what makes this movie such a standout in the action genre that 25 years later a third sequel is coming out and a show based on it aired last season? I think it's the strong dramatic components at its core. Sarah's goal is as primal as it gets - survival. And her obstacles are huge - the villain is called a Terminator and certainly lives up to its name. She's facing a nearly indestructible killing machine.
And perhaps most important, Sarah is someone we care about. Don't under value the scenes in the beginning of the movie where we see her at work as a poor put-upon waitress and at home with her roommate and pet. Sarah's a sweet person, one who isn't having much luck right now. She even gets stood up for a date.
The visual effects were stunning in their day and Arnold is certainly iconic, but it is our identification with Sarah and the threat to her life that makes the movie a classic in the action genre that still thrills today.