Alien was one of those movies I watched as a kid, and just left me breathless. It was amazing, largely in part of the style, and because it had simply not been done before. I remember this movie very fondly, and it's definitely in the top 10 movies that I've seen in my lifetime. That said -
Aliens is sometimes one slot above, and sometimes one slot below Alien on that list, depending on what I'm in the mood for. That was damn near perfection in terms of mood, tempo, tension building, and climactic explosion. (No, that last one is not actually a term, but it is MY term)
Alien 3 was a disappointment for me initially, because of the fact that I'd read the comics, and they had a HUGE departure from them at that point. That was not what I was expecting at all, and considering that Newt and Hicks both had a huge role in the comics... Yeah, them dying offscreen before the movie even started, was more than just a small letdown.
Resurrection... I like it and dislike it for several reasons. I like the cast quite a bit, I like a large portion of the story, but by the time this movie got made, they'd started getting rather schlocky with the whole, "Well, we know that no one else has successfully controlled this insanely dangerous creature, but we will succeed where they failed, because, well, fuck them. They thought they were so smart."
Prometheus is nothing but a half-assed, huge disappointment.
Those first two films, though, were something that actually made me think about a lot of things when I was younger. I still love them both to this day.