The original Hellraiser is about as close as you can get to a perfect horror movie.

Oculus! Great movie.

Cabin In The Woods is also amazing, but I'm not sure it quite qualifies as a "horror movie".

Edit: Oh, I thought of another one. The Pope's Exorcist is good IMO.

The descent.

Unnecessarily scary situation made unnecessarily scarier for the benefit of everyone and no one. Go in without any spoilers. Experience every type of scary.

Sorority House Massacre 2

Super campy low budget b movie. Five sorority sisters spend the night in their new sorority house, the old Hokstedter place, and things start to get dangerous after they use a ouija board.

The movie went from no script to filming completed in seven days. They were in a rush because they only had a stage for the week the studio owner was on vacation. The flashbacks and lore are from Slumber Party Massacre rather than Sorority House Massacre. It was not originally written as a sequel but the distributor thought it would sell better and there was no intent to make a sequel for Sorority House Massacre because, even for low budget horror, it was an exceptionally bad movie.

My favourite? Can't single it out. Horror was the escape for my rather awkward teenage years.

Maybe American Werewolf in London. The soundtrack. The practical fx.

Oh, The Thing. Again, the fx. But the tension.

Or... People Under The Stairs. A horror where, ultimately, capitalism is the monster. Also a dude in a gimp suit is the monster.