The Monster is a machine. They are typically animals of some kind or another, merely searching for their next meal, or for something else they need biologically. The Alien creature from the movie Alien is the perfect example. It doesn't care about the pecking order of the crew of the ship, or about the humans and what their plans are. It is an animal, loosed upon a new environment and doing the only thing it knows how, and that is everything it can to survive. The creature from Alien is so memorable because at the time, nothing had ever been seen like it before. The design of the creature and the way it was shot throughout the movie, mostly in shadow, with brief glimpses of its full form played havoc on the imaginations of anyone who saw the movie. The fact that even when you saw it in full, you weren't seeing everything it could do, through acid blood and the second set of jaws, only added to the magnificence of its form.
Is there a better example of the classic villain than The Joker? Utterly insane, he lives to create problems for the heroes. His schemes may stretch the gamut from simple extortion to threatening to destroy the city, or even the world in a fiery cataclysm, but he does it all for one main reason; to make Batman come out and stop him. In both comics and in cartoons, examples have been given that without Batman, Joker doesn't really even want to commit the crimes that he orchestrates. It's all a game to him, and without enough players, it's just not fun. He demonstrates the main difference between the villain and the monster; motivation. Monsters do what they do just because it's what they do. Villains have much more intelligent motivations, be it world domination, pointless destruction, or a few laughs.
Of course, there is always wiggle room, and in popular language, the two have somewhat become mixed. Villains that commit especially heinous acts may be called monsters, many times in a poor attempt to separate him further from the heroes and make his threat seem larger by comparison. There's a small tip, if you have to resort to wordplay to make your antagonist seem like a larger threat, he probably needs more work. Something to keep in mind is that your heroes (or survivors as the case may be), are only as great as the obstacles they overcome. The greater the antagonist, the more impossible it seems to defeat him and the more times the heroes try and fail, the more satisfying is the finale when he is finally overcome. After the story itself, the antagonist is probably the most important feature in a good Horror story, even more than the protagonist that has to get past them.
I've already gone over two of what I think are the most memorable antagonists, I'd like to bring up a few more before I close out, to give examples of what I think makes them truly memorable.
Sephiroth from Final Fantasy 7 is one of the best villains ever made. He actually grows throughout the story, as you discover more about him and his fall from grace. His motivations seem simple enough at first, your average revenge riff, but as the story progresses you discover his true goals are much deeper, more destructive, and more evil than that. In addition, his physical strength visually grows throughout the game and twists in the plot even give him power over the heroes in ways that you don't see coming and that are truly terrifying. For the game hailed as the rebirth of the franchise, Sephiroth carries much of the responsibility for that.
Freddy Kreuger is also one of the best villains in my opinion. He actually has two motivations, which combine into what he is. He was a child killer even before he was burned to death, something he did out of pleasure, and the retaliation of the parents and community only gave him the added bonus of revenge. His abilities to attack in and control people's dreams is what makes him truly frightening as a villain though. There is nowhere to hide and it doesn't matter how far you run. Nobody can help you. You only face him alone and on his terms. How do you fight something that always has the odds stacked in his favor and relishes the pain and fear you feel because of that? It makes those who survive his rampages (if there are any), all the more impressive in the end, and that, is the hallmark of a truly great and memorable villain.
Who did I leave out? Who are some of the best villains and monsters in your opinions? Feel free to leave comments with your thoughts.