It's the only one that matched Alien in terms of visceral intensity. And whilst I think Alien 3 gave Ripley a great choice to make, at the end of the day, it was two hours of pure victimisation up until the final scene, and in Aliens Ripley makes the decision to fight back not once, but twice, first when she decides to return to Acheron, then when she returns to the hive for Newt. Alien no longer seems complete without Aliens coming after it. Aliens ties up all the themes of Alien. Yes, Alien 3 and Resurrection work as sequels, but they don't match Alien, and admittedly they do have a degree of "this seems tacked on" about them, not least the eggs on the Sulaco debacle.