Well by best horror movie and scariest are 2 different things.
The best is probably The Silence of the Lambs as it is the only true horror movie to win best picture.
The scariest is really going to depend on what hits home on a personal level. When I was a kid, IT really scared the shit out of me. As an adult, The Ring and Conjuring probably are the scariest. Jaws isn't really a horror movie btw.
Yep, scariest isn't the same as best. Scary can help make a horror film be a good horror film.
I agree, The Conjuring with the scene's in the kids room were pretty scary. Wan does it well. And the scene where the long black hair in the face b/w chic comes out the TV is some scary wits from The Ring. Speaking of Wan, Dead Silence has some dead scary scenes too.
I hate to break it to you, but neither IMDB nor Rottentomatoes list The Silence of the Lambs as a horror film... they give it Crime, Drama, Thriller and Mystery & Thriller/Crime respectively. They give Jaws Adventure, Mystery/Thriller, and Holiday/Mystery & Thriller respectively.
I'm not a stickler on defining what a "horror" film is, as it's always subjective. Jaws undisputedly has tons of horror elements in it. It pumps up the visceral terror in lots of scenes... I'd argue making people afraid to go into the water was the kick of Jaws, not the scumbag mayor trying to keep the beaches open.
I think you'll find Horror lists by horror aficionados almost always list Jaws. Whereas studios will avoid it, as horror still carries a "B-movie" stigma to it.
There's lots of films where it's really hard to declare if they're also part of the horror genre or not, like Alien, Gojira, Them, Aliens, Se7en, Predator, Arachnophobia, Shawn of the Dead, Fright Night, Silver Bullet, Ghostbusters, etc.