The mimetic organism in John Carpenter's The Thing is easily the most mysterious character on this list. We never really see The Thing, only the people and animals it absorbs and imitates; the closest we come to a glimpse of the real Thing is in the grotesque states we see it in when its imitations are incomplete or threatened. We also never know what it's thinking - we know The Thing's goal is to assimilate all the lifeforms at the space station and (it's implied) all life on Earth, but we never know its purpose for this. In nature, mimesis is usually a defensive measure, which doesn't make sense for The Thing. While 1951's classic The Thing (From Another World) had a large, hulking alien as a stand-in for our fear of infiltration (Communist or otherwise), the amorphous alien of Carpenter's remake represents a more universal fear of the other. Of all the iconic extraterrestrials in film, The Thing is the most truly alien.
The overwhelming atmosphere of isolation and anxiety that Carpenter carefully builds is aided immensely by Rob Bottin's jaw-dropping makeup effects. The Thing was released in the heyday of latex-powered horror movies, films like An American Werewolf in London and Videodrome that relied on pre-CGI state-of-the-art makeup effects to create convincingly graphic scenes of the human body being transformed and/or mutilated. Bottin had the idea that The Thing would retain a cellular memory of organisms it had previously imitated, which would leave their traces in each stage of transformation. It's a brilliant idea that results in the be-all end-all of creature features - as Vincent Canby described it in his 1982 review in The New York Times:
It's entertaining only if one's needs are met by such sights as those of a head walking around on spiderlike legs; autopsies on dogs and humans in which the innards explode to take on other, not easily identifiable forms; hand severings, immolations, wormlike tentacles that emerge from the mouth of a severed head, or two or more burned bodies fused together to look like spareribs covered with barbecue sauce.Canby means this all as a negative, but if you're a horror fan who somehow hasn't seen The Thing, doesn't that make you wish you were watching it right now? The reviews were very vicious when The Thing was released*, and the film bombed at the box office - it was released two weeks after E.T., and Carpenter has famously observed that Spielberg's film featured an alien that made people cry, whereas his alien made the audience throw up. But while The Thing's content was quite envelope-pushing for its time (it's still very strong), the negative reviews that complained it was an empty gore-fest were way off the mark. The effects are very much at the service of the story, and the strong performances of the ensemble and Carpenter's mastery of framing and timing to maximize suspense (including the most effective moment of misdirection, during the blood test scene, that I've ever seen) are crucial to the film's paranoid atmosphere. The film's cynical, ambiguous final scene suggests that The Thing is best-read as a stand-in for anxiety itself, that dreadful, amorphous monster that might be hiding in the most familiar of places. The tagline for The Thing was highly accurate - man is, indeed, the warmest place to hide.
*The critical consensus has improved a great deal over the years. I was amused, when watching At the Movies, when Roger Ebert recommended skipping the remake and watching Carpenter's film instead; he gave it a thumbs-down at the time.