3. The Alien - Alien (1979)
A wildly popular science fiction-horror crossover, Alien makes the list as the ultimate extraterrestrial monster you most definitely wouldn't want to dream about (although ET freaked me out as a child). Receiving an Oscar for Best Visual Effects, the monster wreaks graphic mayhem upon a spaceship crew in the depths of outer space - it is essentially an outer space stalk and slash with a big, scary monster. The most iconic scene in the film is obviously when spawn of the monster comes bursting out of John Hurt's abdomen. No matter how many times you watch this scene, it still has incredible powers to make you go Eek! That was just the baby alien. Fully grown, the alien launches a spectacular attack upon the rest of the crew that totally slaughters them but for Sigourney Weaver who manages to survive by the skin of her teeth. With creative input from famed Swiss artist of the macabre H R Giger, Alien depicts possibly the scariest and most repulsive alien form in cinema history. Definitely an extra terrestrial to be haunting your dreams.