The website io9 posted a list of the movies from the ALIEN franchise and what a persons favorite says about them. The first movie Alien is perfect in some many ways and is one of the best horror/sci-fi movies ever made (take that 2001), but my favorite has to be the sequel Aliens (another seamlessly perfect movie).
It is the first of the Alien movies I saw when I was a very little kid. My dad was watching it late one night and I joined him because I couldn't sleep and being a kid I always wanted to be near the TV. It did scare me but soon grew on me until a decade later when the Aliens toys came out with the mini comics with it which made it for sure that I'll never give up on this franchise (Alien 3 didn't help).
Aliens also shows the way to change tones in between movies like the shift between the first 2 Terminator movie.
Aliens is my favorite out of the 6 movies (this list came out before Prometheus premiered).
"You believe that high art can be found inside an action film. You appreciate the difference between Predator and Armageddon, and that while both are laden with cheese, long speeches, and a crew of lovable misfits, one is actually a good film (Predator). Although you are more than willing to argue the merits of film like Starship Troopers (and you will win that conversation every time).
Stuck in a firefight, you know the proper cliched characters to surround yourself with so you'll be the last one standing. You believe in shooting first, and asking questions later. But beneath it all beats the heart of a loner just looking for a stoic Michael Biehn-type and a wild child to settle down and start a family with."
If you got a different favorite than me go to the io9 website here and see what it says about you.