Have to say my fave is Big Trouble..., one of the funniest films ever.
I didn't realise Big Trouble... was a John Carpenter movie. In fact, I like all the films I've seen of his. Assault on Precinct 13 comes a really close second.
I remember The Thing being really scary when it first came out. It was talked about non-stop by all the kids around me. I don't think the Sfx have dated well though and it doesn't feel quite so scary and tense watching it now.