Alien - A masterful, perfectly paced thriller. Superb atmosphere, visual design, and sound design. A great cast. The scariest movie of the series, with the most satisfying plot.
Aliens - This is a different kind of movie than Alien. Less a tense and chilling encounter with a mysterious sinister creature, more an in-your-face guns-blazing fight for survival, with a more extroverted cast to match. Ripley really shows her mettle in this one. Many memorable lines and scenes. Personally though, I prefer the former type of film.
Prometheus - Half the characters are imbeciles, but the atmosphere and visuals are superb, in some ways surpassing the original Alien. David is an intriguing and well played character. The pacing and plot are engaging, if you like sci-fi adventures gone wrong as much as I do. Prometheus is a weird conglomerate of a brilliant and gorgeous and absorbing sci-fi film and an unbelievably stupid thriller where people do things like run in a straight line away from a giant rolling object that travels slowly in a predictable direction.
Alien: Resurrection - The tone is weird but the visuals are on a whole other level than Alien 3's. It's a pretty movie with appealing cinematography and good creatures, until you get to the end. Ripley's character is more potent and interesting than she was in Alien 3. The rest of the cast is more charasmatic than Alien 3's. The plot is basic but it's still more engaging than Alien 3's.
Alien 3 - Even the director's cut is trash. The characters and setting are uninteresting. The Xenomorph looks dumb, and is too often shown in full view, running around like some sort of mutant dog in a Resident Evil game. Everything feels lame and low budget. Nearly every aspect of this movie is a letdown, apart from the very beginning where Newt has died. That's a nice and brutal touch. Not even the likes of Sigourney Weaver and Charles Dance could save this film. Oh well.