The Xenomorph in the Alien: Covenant.
While the atmosphere of the first movie and its James Cameron directed-sequel have certainly contributed to the franchise's persistence in pop culture, a lot of it is propelled by the cultural fascination with the titular alien. Based on his painting Necronom IV, Swiss artist H.R. Giger designed the Alien from the ground up, with an emphasis on a fusion of organic and mechanical aesthetics he referred to as "biomechanical." The Alien 3 and the Neomorph in Alien: Covenant. When first confronted with the unknown creature in the original film, the crew of the Nostromo react in horror and disgust as it begins picking them off one by one. However, one member of the crew holds a deep-seated reverence for the Xenomorph, something that helps propel the movie into it's terrifying third-act.
In the original film, Ash is the team's Science Officer, an android masquerading as a human being to further the personal interests of the Weyland-Yutani company. Ripley, particularly, doesn't find this out until much later, and is almost killed by Ash as he attempts to stop her from alerting the rest of the crew. It's heavily implied that Ash was aware that the distress call they answer was related to the Alien, as the communications logs on the ship indicate that the company programmed him to bring the Xenomorph back at the expense of the rest of the crew.
Ash makes several remarks throughout the film aimed at the physiological and biological intricacies of the creature, calling it both "beautiful" and "a perfect organism" at different points. This is in reference to the Alien's perfectly designed structure. A Xenomorph life cycle is impeccable and effective in its own right, and everything about the creature's body is designed to kill; it's mouth and secondary jaw, it's razor-sharp tail, and even it's acidic blood are more than enough to kill most other living creatures. In every way possible, the Alien is a perfectly designed killing machine.
On a deeper level though, Ash's iration of the Alien makes sense in relation to the circumstances of his own existence. In the sequel Aliens, the altruistic android Bishop makes a comment about Ash's model being faulty, a flawed state that's shown by Ash's unhinged attempt to kill Ripley by forcing a rolled magazine down her throat. His programming is already likely affected by the poor quality of his modeling, which explains much of his perceptive state throughout the film. However, beyond any unintentional programming errors, Ash is an android burdened with some semblance of "human" characteristics. In the log that he keeps on the computer's main database, he writes that he's lonely - a condition that androids, for all intents and purposes, shouldn't be able to experience.
His reverence of the creature could also be informed by the fact that the Xenomorph is not an intelligent, feeling species. The Alien's sole purpose in life is to survive - everything else is a non-issue, something that Ash probably envies when compared to his inherently flawed existence. The Alien series has spent the last 40 years exploring the depths of not only the Xenomorph itself, but the question of what it means to be human and what it means to be alive. While the human characters in the franchise struggle to survive, androids like Prometheus' David and Ash struggle with the nature of their own inhibited existence. In Alien, both androids and humans find themselves hindered by the very qualities that define humanity, leaving them vulnerable to the cold and unfeeling bloodlust of the iconic Xenomorph.
