I concluded that I didn't find aliens particularly scary because they're usually portrayed as intelligent, and usually posess some degree of reason. Ghosts and other supernatural beings, however, are usually obsessive and single-minded. For whatever reason, I find this a great deal more frightening.
I think probably the best alien film I ever saw—possibly the best ever made—was The Thing from Another World (1951), otherwise popularly known as The Thing. It had a B-grade cast--Kenneth Tobey as a USAF pilot and ranking military officer at a remote weather station in the frozen north inhabited by military and scientists. Dewey Martin is a NCO crew chief, Douglas Spencer is reporter, Margaret Sheridan provides the love interest to leading man Tobey, and a young beginning actor James Arness as The Thing. What really sets this low-budget film apart is that it’s the only sci-fi film reportedly directed by Howard Hawks. Hawks takes only a production credit for the movie, but as one critic said, “His filmmaking style transcends Christian Nyby's nominal direction: rapid-fire, overlapping dialogue, an ensemble of comrades whose professionalism is tempered by wisecracks, and unsentimental female characters (embodied by feisty romantic interest Margaret Sheridan) recall Hawks's signature works.”
As is the case in all the best horror movies, you never get a good look at the monster, other than to see he’s a really big humanoid type. His human adversaries are far from perfect. After finding a flying saucer embedded in arctic ice (the scene of airmen and scientists forming a big circle around the edge of the vessel they can barely seen under a large sheet of refrozen ice is priceless!), they accidentally destroy the spacecraft while trying to melt the ice with thermal explosives. But they do manage to chop out and take back a block of ice containing one of the aliens who was thrown clear in the crash. In a later encounter, one of the humans falls backwards over a cot and breaks his arm! Meanwhile, the alien seems to survive anything, even an attack by sled dogs that tears off one arm (he then simply grows a replacement).
The film is great at building suspense, as in one scene with Martin uses a geiger counter to follow the Thing’s radioactive trail to a large storage box. As others prepare their weapons, Martin throws open the front panel of the box—and a dead husky falls out!
I was just a kid when I first saw that movie at a Saturday afternoon matinee in a theater chocked full of us rugrats with our boxes of popcorn. And when that dog tumbled out, every kid jumped about a foot out of his seat! Man, popcorn rained down in that theater like an arctic blizzard!!