It's a really tough call for me. Alien was definitive - changing monster and suspense movies forever. The writing, the scene selection - it's all really good. Aliens had such amazing tech. And such lines that hinted at a huge universe! Then again there was AvP which was one of the best b-flicks I've ever seen.
Aliens 3 and Resurrection are forgettable. 3 never quite managed to recapture what (I think) was an attempt at the suspense and terror of the first film; although the dog alien was really cool. I have no idea what they were going for with Resurrection, but ti did not impress.
Promethius was weird. I know it's supposed to be part of the Alien series, but to me it's just a very strange sf movie. I like it but I don't understand it. It certainly had me jumping and quite freaked out at what happens to one of the characters. But it's ponderously slow. The tech is beyond amazing; and it's part of every scene, and every character's plot. Good SF. Odd writing.
Covenant I've not seen.
AvP Requiem was a gore fest of a movie. It lacked the suspense of the first Alien movie, the skill of the first Predator movie, and the fun of the first AvP. It failed as a sequel.