I picked Carrie and all John Carpenter films in the list, because I love them all, have seen them several times over the years, but if the choice had to be just one from the list the only film may well have been Let The Right One In (which is among my selection now too!), because that film has such a tight atmosphere and is a fantastic rope-walk between innocent child film and horror that it is an outstanding addition to the genre. The director (and probably the author who participated in the script writing process AFAIR, did a good job and left out stuff from the novel that might have looked ridiculous on film... Tip: Forget the US remake, it made so many mistakes and lacks the acting finesse of the original!
Edit: I forgot to mention I also picked Silence of the Lambs, those who know the film will know why
