gog2002x: So how much did I really miss out on as far as movies? (this is the bigger question)
I think Aliens 3 is kind of ok on it's own (though certianly not great either). I feel like the worst thing about it is that it kinda ruins Aliens, which is a far superior movie. I haven't seen Resurrection.
The first AvP movie I actually rather like, but if you're hoping for something on the level of the original AvP comic, you'll be disappointed. You have to approach it as a fun, dumb popcorn flick. If you're ok with that, it's probably the third best (or at least most enjoyable) film in both franchises.
AvP 2: Requiem is one of the worst things I've seen in a theatre, and believe me, that is not a statement I make lightly. It basically reduces both franchises to a cheap B-movie teen slasher. And a very, very bad one at that. It's kind of amazing what a low it is for both Aliens and Predator.
Prometheus is an astoundingly stupid movie, but it was a lot of fun to watch because the contrast between how dumb it is, and how seriously it takes itself makes it quite hilarious. Plus, it is a damn nice looking movie with some very good music. Still, I never felt inclined to watch the follow-up, Covenant.
I know this will sound insane to most people, but really for my money the best entry in the Alien series after Aliens franchise was... the Superman vs Aliens comics. It shouldn't work, but it does. It really is a pity Disney and not Warner now owns Aliens, because an adaptation of that, even only in animation, could have been jaw-dropping.