I love Alien; its easily my favorite of the films. Its pacing is near perfect, I like all of the characters and even though I know all of their fates I'm always engaged in what they're doing.

I like Aliens. Its definitely and adrenaline pumped film. In many ways that was part of what made it work; it didn't try to be as scary as Alien (or claustrophobic or shocking or however you want to describe the firs film) and thus became its own film. A large cast gives less room for many characters getting characterization though (but certainly by the middle the surviving characters have our interest)

Alien³ is probably the film I feel is the weakest. The story just isn't there to make for compelling viewing. Its essentially mirroring the first film though, but without the compelling characters to draw you into the narrative.

Alien: Resurrection is a film I've enjoyed since the first watching. As the third film mirrors the first, the fourth mirrors the second; the twist being mercenaries instead of marines. I think the characters are fun. I do think the final confrontation isn't as strong as it should have been which somewhat deflates the experience with the film (even though I don't like the film as much as the others, the end of the third film definitely delivers on its concepts)

I think the first two films are connected. The third film - which summarily excises any connection to the second - disconnected me as a viewer (although clearly tied into a direct through story from the first film).

The fourth film is its own thing; it doesn't have a direct link to the other films (this Ripley isn't the "real" Ripley). The Aliens vs Predator films likewise only have the loosest connection to the original film (and frankly IMO are fairly rubbish; the second AvP film being one of the worst films I've ever seen).