What is your favourite "alien monster" and why?

There are so many... But the Alien Xeno has to be the most iconic, to me anyway. While i love the Predators as far as a threat goes the Xeno and a Xeno Infestation is something really bad. But as a Weapon its flawed, its only use is drop onto a World and Watch it invest and take over that World, its basically like using a Nuke or a Disease that changes the population into Zombies etc..... But the most fearful has to be The Thing..... [b]The Thing[/b] is simply the ultimate in terrifying and unstoppable Killing Machine/Organism. Suffice to say drop a 1000 Xenos or a Million on the USA would be bad... But just a single Thing outbreak and infection in a populated City like New Work would soon spread across the USA and the World. The Hidden and Unknown Horror. If any movie Monster Alien was to be real and a real threat i cant think of anything as terrifying as [b]The Thing.[/b]