Aliens made me a fan of the Alien franchise. Even though my favorite film is Alien, I found it too scary to watch when I was younger, finally getting around to watching it when I was around eleven or twelve years old. I have remained a huge fan of the first two films since then, and I still quite like Alien 3. I don't like Alien: Resurrection as much I a used to, but I still watch it whenever I watch the other three. I really like Prometheus. I think it's a good film, but a film that left much to be desired. While the characters are under-written and come off far less realistic than anyone from the first three Alien films, I find the blow is softened by the superb visuals and promise of future films in the series. Even though it's not a movie, Alien Isolation is a game I love so much that I will probably play every time I finish watching Alien, and then continue on watching Aliens after beating the game.
Predator is what made me a fan of the Predator franchise. I've never liked Predator 2 very much, but I found enjoyment out of watching it and still do. I much prefer Predators, which I really enjoyed and still do, but Predator still remain my favorite of the franchise.
I'm not a big fan of AVP: Alien vs. Predator or AVPR: Aliens vs. Predator - Requiem, but like the weakest films of the Alien and Predator franchises, I can still find enjoyment to be had out of these two, more so the first film than the second, which I consider to be weakest out of all the films.