I'm taking my two right now. These are films I really enjoy, one is in my top films list, but I just don't think they belong in a horror best list.
Les Diaboliques (1955)-- I just watched it and will give a review later. But this isn't a horror. And except for a few moments near the end, it never feels like a horror. A good film, needing to be watched, but not a horror.
Silence of the Lambs-- I love this film, and there are some truly frightening moments. But what makes this film is that it is a crime drama with horror elements. I would say that it is much more a psychological thriller.
And I am very much in support of Martin's first pick, Aliens. An action film with some very scary moments, but not a horror film. Unlike Alien, which is one of the great horror films of all time.