Alien: A very good movie
Aliens: A very reasonable movie, and I, personally, think it followed pretty well with the mythology that was established in teh first movie
Alien3: This is where it started going down hill, but it could still be a passible ending to Ripley's involvment in any and all ways.
Alien 4: Total crap
Predator: A pretty descent movie and well handled for the most part. I could have done without Ahnold, but considering when it came out, if he hadn't been involved, it might not have been made.
Predator 2: It made sense, but it was poorly done.
AVP: More total crap, I mean, like seriously shitting through a screen door kind of crap. I'm not opposed to the idea that the Predators would set up a hive, but they are the reason human civilization became advanced? Nope. In fact, I have issues with any AVP movies taking place on earth, or having human dialouge for that matter
AVP2: Didn't even attempt to go and see it.
i've read a couple of the Predator books, and they weren't too bad. I even have a number of the Aliens and Predator Dark Horse Comics, and they're pretty much all superior to any movie released since Alien 3.