I haven't seen the other two, but I liked the first one. Thankfully I never read the book and could get worked up about how it's not like that.
But the combination that w that we're forced to root either for faschists or for human-murdering bugs, and that is mixed with gratious violence and nudity is difficult to resist for me.
I am also a big fan of the fourth Alien movie.
I like the bad-ass Ripley and the bad-ass crew (I think you can see that Joss Whedon wrote the initial script, at least if you've seen Firefly). I like how the aliens figure out how to escape their imprisonment and really appear "smart". About the only bad part is the stuff with the Alien Baby feeling love for its human mother and being sucked into vacuum through a tiny hole in the ship. :p