The sequal was pretty good. James Cameron did a great job making a big-budget sci-fi film. It was enjoyable, to be sure, but it didn't have the "terror" factor and uncomfortable sense of dread that Ridley Scott's first film did.
Alien was a horror film.
Aliens was a sci-fi film.
I've yet to see the last two.
I'll correct myself here:
Aliens was an action film.
"I've yet to see the last two."
Actually 3 if you include AVP. Total of 5 with that one.