I highly recommend the Spanish found-footage film [REC], and another vote for Cronenberg's The Fly. I just rewatched it the other day and it's fantastic. The script is terrific, the effects are awesome, and the story and pacing are great. No good vs evil or supernatural elements, just a gradual descent into darkness and body horror. I love the 'insect politics' monologue.

I wanted to mention REC but forgot. I normally hate the found-footage genre but this is an exception. The sequel is also good.

AMEN
Of course when it comes to scary/disturbing with C. Thomas Howell I think it's a toss up between Soul Man and Side Out. :P

Indeed, I feel the same way. =)

That expalins why the movie felt like an old school horror movie that was never made in the 70s until now.

Is this a serious thread or can I post?

The granny scene from Legion:
https://www.youtube.com/watch?v=MAO6TmGnZ9s

"I'm a fucking Nigerian!" from La Horde:
https://www.youtube.com/watch?v=ChqXF84d3Jg

Don't watch either link if you're minding small children.

All a matter of taste, of course. I could post dozens of suggestions depending on what you're looking for exactly.
For me, a must-see horror movie is something that 1.) is first and foremost a good movie and 2.) has some scare to it. As mentioned, Stuart Gordon is a good consistent director. If you like Argento style or Cronenberg style, go for those. Saying that, here's my list, some repeated, some not yet mentioned:

The Shining (best horror movie, ever, IMO.)
Ravenous
NightBreed
Jacob's Ladder
Event Horizon
Lord of Illusions
The Entity (80s)
Pulse (80s)
Possession (sam neill)
The Sentinel (chris sarandon)
Martin
The Unknown

not horror but has a high creep factor:

Lost Highway
The Wicker Man (original)
Silkwood

Fulci's The Beyond, Zombi 2 and City of the living dead
Argento's The Three Mothers Trilogy (Suspiria/Inferno/Mother Of Tears)

Brilliant :)

Good call, I meant to mean the other one, Dawn of the Dead, somehow my memory mixed the order up.
I actually like all of Romero's zombie films, even though some are weaker than others.

All other movies on my list - like The Thing - refer to the older versions.
With Invasion of the Body Snatchers I refer to the 1978 film, with Donald Sutherland.
I still think its one of the creepiest movies I've seen.

With Ring and Ju-on I refer to the Japanese films, although the American versions were also great.

Yepp, that's the one I meant, with good old Rutger, he is perfect in that role.

Definitely agree, I liked both of those films.

This might be silly, but The Blair Witch Project really freaked me out the first time I saw it.
This was before the whole documentary-horror style became popular.

Also, Paranormal Activity had the same effect one me.

Into the Mirror, a South Korean film, was very good. They did a Western version with Keifer Sutherland.

Whistle and I'll Come to You is a short but creepy black & white film from 1968.
It is very minimalistic and slow but I thought it was great this way.

Yeah that really didn't jibe with me. The rest of the film has some merit though.

The Descent was pretty good. Good use of its limited budget and the women are not screaming dumbasses.

The Exorcist scared me shitless once upon a time (Film and games just cant scare anymore, maybe its my age)

Mood helps. And this is a long stupid story, so sit down.

I have seen The Shining a few thousand times over the years to the point where I almost found the movie comical, not a chill to be had. Then a few years ago here I am taking care of my B&B by myself (the rest of the family was down in Florida) when a blizzard hit. So it's middle of the night, I'm alone in an inn, in the middle of nowhere, during a blizzard, and The Shining comes on. Needless to say watching it then and there was a huge mistake because I spent the next week jumping at anything that moved, afraid to so much as leave my room for fear of what might be waiting for me. :P

Indeed, kind of cool when you think about it.

lol reminds when I used to work as a concierge. Id watch zombie movies and imagine how I would could barricade the building and how long I could hold out.