Job description: The MGMFOS has one primary function, and that is to creep us out so comprehensively we'll be looking under the bed and sleeping with the light on for weeks. When a director gets it right, a truly believable space alien at once captivates and terrifies us, firing the imagination as to what horrors might really be lurking out there, in a galaxy not so far, far away. Most recently seen in: War of the Worlds. Tom Cruise and Steven Spielberg, a winning combination in Minority Report, team up again for another sci-fi tour de force, reviving H.G. Wells' tale of invading Martians. Spielberg's aliens were kept under tight wraps ahead of the film's premiere, but we now know what he was hiding: huge, slimy, tentacled, tripod-mounted beasts with supremely bad intentions, who bring with them a putrid red weed that oozes its way across the landscape. Defining moment: Cruise fights his way through the red weed and laser beams into the dark heart of the alien mothership, only to find John Travolta in full Battlefield Earth regalia. They join forces and conquer the planet in the name of Scientology. Just kidding. Classics of the genre: I'm partial to Predator, the towering, mandible-mouthed intergalactic Great Green Hunter who dukes it out in the jungle with The Arnold in a classic sci-fi duel. John Carpenter's The Thing is also up there in the hideous stakes - a murderous creature that can assume any form, runs amok on an Arctic outpost and does horrible things to huskies. It's hard to beat Tim Burton's brain-baring, goggle-eyed little green men for comedic relief in Mars Attacks! Ultimate avatar: Nothing holds a candle to Ridley Scott's Alien, a truly nightmarish vision of malevolent intelligence and unstoppable instinct. Increasingly silly sequels, including an execrable Nintendo-esque duel with the Predator, have done nothing to dull the edge of this classic monster. From the face-hugger to the gory chest-burster moment, Alien is an unparalleled study in shock-studded suspense. Artist H.R. Geiger's creature is pure nightmare, a killing machine with acid for blood, a phallic symbol for a head, and a set of choppers inspired by a Sigmund Freud concept, vagina dentata (a vagina with teeth). Not to be confused with: ET, Kermit the Frog, Jamiroquai.