My personal ranking from best to worst:
Alien
Aliens
Predator
Prey
Alien 3
Alien Resurrection
Alien Covenant
The Predator
Predator 2
Prometheus
Predators
Aliens vs. Predator 2
Aliens vs. Predator
Number one is obvious for me, all Three top movies are untouchable classics, but first Alien is simply my favourite movie of all time. But I think original Alien quadrilogy haven't a bad movie in it, they were all great and very different. And I will always defend Resurrection, it's underrated B-movie masterclass. With Predator movies it's more cmplicated thing, before Prey I think this franchise had only one trulli great movie, now it has two. Predator 2 is just fine, Predators is boring and unimaginitive product and The Predator I actually like, as this silly, dumb fun mockery of 80' sci fi movies. Covenant has many problems, but also as many great thing and ideas. Prometheus is a bad movie, but interesting one at least. And AvP movies... Yeah, I don't talk about them.