The 1982 movie The Thing is one of my favorite sci-fi/horror movies – right up there with Aliens. It a paranoia fueled mystery of a alien organism that can assimilate and imitate both human or animal hosts.
Based on the 1938 story Who Goes There by John W. Campbell Jr, The Thing takes us along into the fear and paranoia of an isolated Antarctic research station with the characters trying to ascertain whether everyone is who they say they are.
What makes it so re-watchable is that the timeline when certain characters are assimilated is unclear. Filmmaker James Cameron even used stand-ins so that the shadows were not easily identifiable. This has lead to years of speculation and fan theories (Clothing continuity theory, Molotov whiskey theory, No breath theory, Who sabotaged the blood bank) about what happened when.
Of all of them, I think the Den of Geek does the best breakdown of the most likely order that each of the characters succumbs to the thing.