My favorite antagonists in science fiction are the inhuman relics of a long forgotten war between two now dead races: the robotic "Berserkers" created by Fred Saberhagen. Originally created to provide the opposition for the human protagonist in the short story Without a Thought, these agents of destruction ended up featuring in several novels and a couple dozen pieces of short fiction - sufficient to fill seventeen books worth of pages.
The Berserkers were created during the same time period as Earth's Paleolithic era by a race now only known as the Builders. They were intended to help the Builders prosecute a galaxy spanning conflict against their rivals, known as the Red Race. To this end, they made the Berserkers engines of pure destruction, making them self-aware and programming them with instructions to "destroy life". Unfortunately, the Berserkers worked too well. They won the war for the Builders, wiping out the Red Race in the process, but then they turned on their own creators and destroyed them as well. Millennia later, the Berserkers still travel through space obeying their long gone master's one command, and humanity and its allies must desperately fight for survival.
The Berserkers are a fantastic antagonist because they are both infinitely malleable, and firmly fixed in their implacable desire to eradicate life. Some humans bargain with the Berserkers, becoming "goodlife", and work for the Berserkers hoping to stave off their own deaths for a while. But they know that their reprieve is only temporary, because the Berserkers know no mercy, no pity, and no kindness, but are driven by their unchangeable homicidal programming. But they are infinitely malleable as antagonists because they can take almost any form - from gigantic starfaring ships the size of large asteroids, to human sized (and shaped) entities, and almost anything else one could conceive of. They are, in many ways, the perfect adversary, unreasonably hostile and unswervable in their dedication to their mission, and provide the perfect antagonist for fallible, ingenious, resourceful humans.
Go to Day 16: What Genre Novel Has the Most Intriguing Plot?
Go to Day 18: Who Is Your Favorite Protagonist in a Genre Novel?