It's really hard for me to rank them in a meaningful way. The mind seeks different things at different times. Even the ones that overall aren't executed as well as I would like still have fantastic elements in them. There isn't a single mainline Alien movie I truly dislike.
When I was younger, Aliens was definitely my favorite. It's just so fun and intense, and the Queen is a showstopper. It's still a great movie and always a blast to rewatch, but Alien3 (which I never disliked, even though I felt obligated to always preface my opinion with, "It's not a good sequel, but...") has gradually taken over the top stop, year by year.
Struggling with mental illness, having my plans for adulthood laid to waste over and over again, Alien3 weirdly felt like it "got" me somehow in its darkly operatic blend of despair and hope. "You've been in my life so long, I can't remember anything else", and all that. My mind has found an intense kinship with these sorts of filmatic allegories (Evangelion is similar), and though I know they aren't for everyone, I am very glad stories like these exist.
A couple others I expect will be unambiguously near the top of my ranking before much longer are Covenant and Romulus. Covenant is another one that, like Alien3, one feels almost obligated to preface any kind words about, but I think I will learn my lesson sooner this time. I think it was the first time I spent a significant amount of time "sperging out" with maximum intensity over a specific Alien film; the set-dressing for David's laboratory left me enrapt, and you may (or may not) remember me transcribing as much of the text on the drawings as possible between two imgur posts. That's just one element of the film, of course, and perhaps it's not enough to outweigh the film's weaknesses, but, I don't know... There is something about the "vibe" that just clicks for me. Hanging out here and bathing in Nightmare Asylum's refreshingly unapologetic love for Covenant is helping me realize, I think, that "yes, I actually like this one quite a lot and I need to explore this feeling more".
With Romulus, it's perhaps too early to tell, but so far the film simply works for me. The much-discussed low points are simply insignificant compared to everything that it does right. Some of the big contributors here (but not the only ones) are the themes of disability, improvement, and selfhood. Andy feels like a character who was made for me, and hopefully many others feel the same. (I think sometimes it's perceived as a discredited trope to made "coded ASD" characters things like androids, aliens, and so on; but in my own experience, that degree of separation from "literal humanity" makes it much more likely that I'll be able to see myself in the character.) Other aspects of the film are a lovely bonus on top of that.
