John Carpenter's Halloween is widely regarded as one of the most influential movies in the horror genre for numerous reasons. Part of what makes the movie so effective is the simplicity in the shots and overall lack of extravagant special effects. This made it more of a psychological thriller than a gaudy slasher flick.
But visually speaking, the indelible image is that of the blank, expressionless mask the killer, Michael Myers, wears as he murders his way through the cast. Aside from perhaps Jason's hockey mask or Freddy Krueger's fedora/bladed glove combo, there's not a more iconic bit of horror villain paraphernalia out there. Carpenter, being the creative genius he is, went out and stretched his budget of $200,000 any way he could, even if that meant springing for ultra-cheap costumes.
In what proved to be his smartest bit of cost-cutting, he didn't even have the prop guys make a unique mask for his villain. Instead, they spent a few bucks on store-bought Captain Kirk masks, painted them white, removed the 70s-in-space sideburns, and widened the eyeholes for added peripheral vision for the actor (and extra nightmare fuel for the audience).