For me it was Predator on TV, i was waaay too young (maybe 5 years old, lol the irresponsible family i got) and was genuinely terrified ! But at the same time fascinated, and this only kept growing as i was too, leading me to this day and gladly being part of this forum !
For the Alien franchise it's a bit more messy, because i saw the films in complete disorder : the first half of Aliens on cable TV when i was like 8 (watched it with my brother but he kicked me out 'cause his goddamn football match was about to begin !), Alien 3 next, taped on VHS, probably 1 or 2 years after Aliens first half lol, then Alien taped on VHS about 1 year after that, and finally the second half of Aliens taped on VHS and Alien Resurrection with the official VHS on sale at the time.