The 1979 movie Alien and its 1986 sequel Aliens are both masterpieces followed by other, lesser sequels. People have been arguing for decades over which one was the better movie. It’s really not a competition, because they are completely different types of movies. Despite following the same main character, despite the sequel following chronologically in the same universe, and despite the same monster species in both, they are movies that occupy different categories. Alien was a horror film, and Aliens was an action movie. And for the story they told, it works.
Because Aliens is so transparently an action movie, you will come across people who will tell you that it’s bullshit, that it’s a violation of everything that made Alien great. Do not trust these people. They are wrong. Aliens is about as perfect as a sequel can be, in part because it’s so distinct from the original Alien. The two movies share a central hero (though Ripley doesn’t really emerge as the hero of Alien until more than halfway through), and a central creeping terror (the incredible H.R. Giger-designed xenomorph drooling-bug monster). They’re both built on the inherent, body-level revulsion at the idea that there’s something unknown in the dark, something that means you harm. But Aliens does what sequels, ideally, are supposed to do. It never retells the original story. Instead, it expands on that story, introduces new characters and new wrinkles, and uses that original basis to tell an entirely different kind of story.
The A.V. Club gives us a thorough breakdown of what made Aliens so good, despite being a sequel. It’s a part of their ongoing series on the history of (cinematic) violence.