It's simple, because of Blomkamp's new film we simply get separate timelines.
Alien, Aliens, Alien3, A:R.
Or
Prometheus, Prometheus Sequel(s) Alien, Aliens, Neill Blompkamp's Alien Sequel(s)
(I personally separate Prometheus from the originals due to Alien V's apparent link to Prometheus 2.)
Or
Predator, Predator 2, AVP, AVP:R, Alien, Aliens.
Or
Predator, Predator 2, Predator 3 (Shane Black), Predators, Prometheus (& Sequels) Alien, Aliens, Neill Blompkamp's Alien (& Sequels)
That's how I personally divide them up.
I can't wait for Neill Blompkamp's Alien and I'm so glad the "Everything is canon." Argument is 100% dead.