I love Alien, it will always be one of my top 5 films (irrespective of genre) BUT it has NEVER scared me. I love the atmosphere of the film, the way it draws you in, its beauty, the tension, the character interaction, but it has simply never scared me, disturbed me or even grossed me out. The film is a fantastic work of story-telling, visual and audio art and I love it to bits (must have watched it at least a couple of dozen times since it was released).
So what has given me a fright? Things that are suggested or implied, psychological horror or the supernatural, such as excellent films like The Changeling or The Woman in Black, but they must have a great story, writing, acting, direction, etc, etc, etc.
Films like The Descent, Saw, etc bore me to tears.