I'll start by saying that as scary as you might think
Resident Evil is I wouldn't class it as a horror film. (I know that the game is survival horror, don't point that out to me

.) Either is Predator. I'd class it as more of a Sci-Fi/Action flick. But anyhow. Doesn't really matter. If it scares you; it's all good.
In no specific order
Alien - dark, brooding and rather visceral. So enjoyably scary due to the great soundtrack provided.
The Shining - pretty much the best horror film I've personally ever seen. Again a very haunting soundtrack adds to the clean and glaring manevolence of Kubrick's masterpiece.
Scream - Quick, and very witty. The first time I watched it I have to admit that I hadn't a clue who the killer was. A very interesting semi-satirical film on the horror genre itself.
Dog Soldiers - One of the best films I've seen in my lifetime and one to revive the horror genre again for the better. It's the Scream of the 2000's. Quick, with some of the best lines I've heard in ages. The charm and overall wit of this film is emmense. Some very good werewolves bring in very good scare value.
Evil Dead - For those that have seen it; and liked it...enough said. Otherwise a very sharp and strong film. Basically Sam Raimi knew enough about film-making that he realised; not always showing us what we don't want to see anyway is sometimes the most chilling of all. Much like in the way Ridley Scott didn't really show us a lot of the alien.
That's about right I think.