Aliens

In the final 20 minutes or so of Aliens, series protagonist Ripley (an Oscar-nominated Sigourney Weaver) prepares to enter the creatures' hive to rescue Newt (Carrie Henn), a little girl captured by the nasty creatures.  Ripley loads up with a massive pulse rifle, a flamethrower, and grenades. 

As the elevator descends to what could be her death, we see how strung out and exhausted Ripley is, covered in sweat and grime. With her weapons ready, she closes her eyes, mentally preparing herself for the horrors she's about to confront.

That little moment demonstrates why Aliens is a worthy follow-up to the original Alien. It expands both the characters and the narrative instead of repeating the same plot and because writer-director James Cameron gives Ripley something sci fi and horror characters don't always receive: choice. 

Many science fiction horror tales drop their protagonists in a pressure cooker and watch them react, and the best ones, like Alien, show us what those characters are made of. In Aliens, we get that, but some of the best moments occur not when the monsters unexpectedly attack but when Ripley chooses to confront them. Instead of seeing her as a fool, we the viewer understand, sympathize, and root for her for doing so.

The Abyss

An early image in The Abyss captures the mystery and danger of the hidden world lurking in the ocean depths. Oil rig workers, recruited by the Navy for a rescue mission, search the flooded innards of a sunken nuclear submarine and are greeted by the pale corpses of doomed sailors as they navigate in clunky scuba gear and breathing apparatus. One rig worker sees spider-like crabs nestled on a dead man's face, and in a gruesome moment, a crab scuttles out of the mouth. 

Even without aliens or creatures of unknown origin, the ocean contains its share of the strange, the uncanny, and the dangerous. Never mind the sea creatures that will eat you without a moment's thought; the human body is not capable of surviving in the ocean without extreme measures and hard-developed technology. With the cold, the water pressure, and lack of air, humans are outmatched and out of their element.