This is absolutely not a dig at the OP, but I'll never understand "which order should I watch" threads, in the same vein as "what is a good jumping on point for (insert tv show here)?" - the answer always will be (and always should be) "the beginning." Any prequel chronologically always requires some knowledge of the movie or tv show that came before it, you will simply ruin the experience any other way.
So (to me) there's only two acceptable ways to view: Either release order of each movie (as if you grew up with them and saw each one as they arrived) or by individual franchise in release order (the latter is my preference, as the franchises shouldn't mix). Like with the release order, I'd always go for first versions of the movies released too, and then you have the joy of seeing the extended/directors cuts on a later viewing. However, if you are set on only one version of each movie, this would be my suggested viewing order to get the most out of the experience:
(Alien Movies)
Alien (Director's Cut)
Aliens (Special Edition)
Alien 3 (Assembly Cut)
Alien Resurrection (Theatrical Cut *you can take or leave this one, I treat it as expanded media like a dark horse comic - Alien is a Trilogy in my view)
(Pathogen Movies)
Prometheus (3D - as theatrically intended)
Alien Covenant
(Predator Movies)
Predator
Predator 2
Prey (Comanche Language Version)
*do yourself a favour and disregard Predators and the Predator*
(AvP Movies)
*worth a one time watch as popcorn flicks /live action dark horse comics, then put them in the same bin as Predators and the predator*
