After more or less enjoying Prometheus I've been watching the Alien movies, excepting the Alien Vs Predator movies because enough is enough.
Part of what makes the original Alien so great is that it's full of unfolding mystery. The alien keeps shifting, revealing new aspects of itself and new dangers. You can't pin it down. It's death itself, lethal, heartless, mindless; yet crafty, unpredictable, and unstoppable. The lesson is that the cosmos is not exactly full of love for the Human, and you shouldn't mess with some things.
Aliens retorts that you can mess with any damn thing you please, so LOCK AND LOAD. Director James Cameron, the Vincente Minnelli of monster movies, replaces the tension engendered by the polymorphing parasite's continual astonishments with the suspense of "Who will survive and what will be left of them?" That suspense was part of the first film, too, but here it's the mainspring. There's a bit of an addition to the aliens' characteristics, but it's more a reasonable extrapolation of what we knew (aliens have mommies too) than the nightmare twists of the original. Furthermore, the escalation from alien to aliens robs the creature of its dreadful uniqueness (though it didn't need to; such are the urgencies of the action genre), reducing the Cosmic Godling to creepy cannon fodder.
Alien3 offers no surprises regarding the alien creature itself, other than that it can look awfully unthreatening in 90s CGI that resembles a Colorform slapped onto the cinematic image. Also, the Alien's POV resembles an old camcorder with dubious filters. Director David Fincher would soon do much better.
The surprises in the film (aside from "Boo!" surprises) come from the convicts with whom heroine Ripley must work; their alien religious motivations replace the unfolding of Alien characteristics. The alien itself is thoroughly banalized by now; the Lovecraftian awe of the original film is reduced to "There's a rabid hippopotamus loose in the high school!" Each of these films has fine ensemble work, but all the sequels feature overt thesping, while the original was nearly Altmanesque in its casual approach to line readings, etc.
At one point in the third film, a guy refers to the alien as a dragon, which inspired me to spend the rest of the film pretending the alien is actually Pete's Dragon.
This exponentially boosted the film's entertainment value.
Alien Resurrection, the last of the Sigourney Alien films, has the look of a particularly glossy Heavy Metal magazine comic. The unfolding mystery in this film is as much about Ripley, newly reborn from hybrid human and alien DNA, as it is about the pureblood beasts. Sigourney Weaver's cool delight in playing a bad-girl version of Ripley is one of the chief joys I found in this film. Wynona Ryder is, as always, fun to look at, but her bitter-pixy appearance can't quite compensate for her undistinguished voice work, especially in the context of her costars. Sigourney and Brad Dourif have marvelous vocal instruments, next to which poor pretty Wynona sounds like a randomly selected person off the street. If only she'd brought back her comical British accent from Dracula! Brad Dourif is one of the few actors who could pull off his scenes of sensuosity with an Alien, though that's just a warmup for later developments.
Underwater aliens are fun, and we get them here, but the real return of Alien mystery comes when Ripley is lowered into a pit of distinct yet inchoate Alien anatomy in a glorious homage to Alien designer H. R. Giger's grotesque tableaus (too bad he didn't get a check or a credit for this film tho) and proceeds to have sex with an alien. It's all presented with White-Elephant arty tastefulness, since who could bear to watch a hard-core version of such a thing? Somebody, probably, but no one I want to have lunch with. The death of the offspring of this union depressed me; as cloying as the creature was, it reminded me of the death of Tink; a creature that never had a chance at life, put to death to spare it further misery.
I didn't like the idea of humans and aliens merging and mating; it further undercuts the evident theme of the original film, that cosmic indifference trumps manifest destiny. This film returns humanity to the center of the universe by showing that we can harmonize with the Alien, but better still if we kill the product of our miscegenation (not the first time inadvertent racism has crept into the franchise, as the many critics of Aliens' black-mother-with-too-many-kids can attest).
One more thing; most of the ensemble in this film seems to be a dry run for screenwriter Joss Whedon's nerd-favorite show Firefly. I thought Firefly seemed like an advertisement for its own adorableness, so with this film I enjoyed pretending that it really was the crew of the Serenity getting gutted by monsters. It's a sad testimony that I have to pretend absurd things while watching films that are meant to take the burden of that responsibility off my shoulders, but that's franchise entertainment for you.
Prometheus, whatever its flaws, put the mystery back into the story by showing us Aliens that sprang from alternate hybrid pairings and, aside from certain basic hungers, are not like the on-model, canonical, in-continuity aliens that had become as exhausted as any other franchised monster. Since the sense of wonder I experienced when the aliens came out looking wrong and strange and new is what I want from Science Fiction, I'm going to say that, for my purposes, Prometheus is the best of the Alien sequels.