The creation scooped 34% of the vote in the survey by the Sky Movies Sci-Fi & Horror channel to mark its Creature Features season.
The Creeper from teen horror movie Jeepers Creepers took second place with 14%, with the shark from Jaws finishing third with 10%.
2,500 movie fans answered the call to find the creepiest creature, with most of the top ten using traditional special effects rather than CGI.
Sky Movies director Ian Lewis said: "There's no doubt that computer-generated creatures look fantastic but this poll would suggest that traditional prosthetic models are still the scariest. With the continuing advances in CGI, I'd be interested to see if new creations like the Cloverfield monster will join the ranks of these classic movie monsters."
John Landis, who directed An American Werewolf in London, said: "I am delighted to see that the Werewolf is still scaring audiences after all these years. I have to admit that movie monsters do not scare me, people scare me. I know of no one stepped on by a giant dinosaur or murdered by a creature from outer space. People tend to be the ones killing people. That's scary!"
The top ten scariest monsters in full:
1. The Alien (Alien, 1979)
2. The Creeper (Jeepers Creepers, 2001)
3. Jaws (Jaws, 1975)
4. Chucky (Child's Play, 1988)
5. Pinhead (Hellraiser, 1987)
6. The Crawlers (The Descent, 2005)
7. Werewolf (An American Werewolf in London, 1981)
8. The Pale Man (Pan's Labyrinth, 2006)
9. Brundlefly (The Fly, 1986)
10. Godzilla (Godzilla, 1998)