Sorry, I should say that the majority that have watched Alien since 1979 is far greater than those that saw it in 1979; as that's what I meant.
Narratively, thematically I think A3 and Covenant are the most powerful.
As for pure horror, I think Aliens' chestburster tops Alien, the ripping and tearing is far more visceral to me, the sounds of the bones snapping as well as the psychology of its' effect on our protagonist.