Had there been a proper space set horror prior to Alien? its a genre defining film, and very well made. The casting is excellent with a variety of actors who all play their role well, and by having only a small cast not one character is wasted.
There are tense scenes in here, and there are shock moments too with plenty of gore like any decent horror. Also the alien itself is cleverley done, it kills as it is "born" and it develops in size and intelligence and threat as the film itself develops. It is no wonder this alien has etched itself its own career.
Also, its got Harry Dean Stanton in it, one of my favourite unsung acting heroes.....brings my count up to ten overall with possibly two or three more(hopefully).
On the original v sequel discussion, all my choices have been the first ones, there is just "something" about the inspiration for a first movie in a trilogy or whatever, but thats just opinion of course.