Two of Fox’s signature franchises seemed destined to intertwine and become linked as one series; both original instalments subverted their specific genres - Alien (1977) the horror movie, Predator (1986) the action movie. They also created landmark central creatures and took B-movie cliches and elevated them to A-list standards.
Dark Horse comics held publishing rights to both properties and after having massive success with each, decided to pit them together in a landmark 1989 mini-series. The response was so massive 20th Century Fox decided to throw an official nod to it in Predator 2 (1990) with the Alien skull sitting on the Predator's trophy mantel.
Certainly, its been a bumpy road for both franchises since their initial classic entries yet each movie has an enthusiast filmmaker behind it, attempting to stake out an individual stamp on the franchise - whether they tried and failed, there is still always a genuine pulse pumping in the film's vision - which is rare for two tentpoles studio franchises.
Staking out a clear-cut order for these films is a high-task, every fan has their own distinct assortment, even with what stands at number one. So this all comes down to a strong personal preference - although I'm sure everyone will agree with my bottom entry.