What is a horror tension scare / visual scare movie is actually scary?

Alien. It's the pinnacle of horror movies. And what makes it so good is the believability of it all. There is no supernatural, no ghosts, no masked mass murderers, no silly "monsters" that defy all the laws of biology and physics.

For “scare” here, I don’t mean I got suckered by a jump scare. I mean the scare stayed with me long after the movie ended - sometimes for months.

Two come to mind although I’m sure there’s more. One is Nightmare on Elm Street. I saw the first one as a kid when it came out, and I thought it was genuinely scary. I’ve been a horror fan since childhood but that was one of the first visual scare movies that actually scared me. I had watched the slasher films that became franchises, and Freddy went there in the very next movie, but that first one was solid (plus I was young and it was a creepy idea).

Since you also mention tension I’ll go in the other direction and say Blair Witch. I was completely freaked out when I walked out of the theater. I watched it cold - no prior knowledge. It was close to opening weekend and I liked the poster. I hadn’t heard any hype. That movie both established that style of filmmaking and has yet to be equaled in my opinion.

I probably watch more horror than any other genre, and I don’t do it to get scared. I enjoy the humor, I enjoy the stories, and I enjoy the effects. I will rarely get caught by jump scares (although I might if the person next to me yells), and I almost never get actually scared. I can acknowledge when a story is creepy-cool (like Babadook) or creep-because-the-metaphor-is-too-on-the-nose like Pontypool, but for the most part I just enjoy them.

The Mist. The real monsters aren’t the ones in the mist and the things we do because we have to become the things we regret the most.

So any kind of buildup of moral and societal decay where people do more and more terrible things because it’s right or good or necessary while you watch helplessly… that tends to get me.

For example, the MASH finale with the chicken. Man that still gets me and it’s been years.

Not sure if this is what you're looking for, but my favorite eerie/creepy horror movie that doesn't rely on jump scares or gore is The Mothman Prophecies. It's about a journalist who stumbles across a small town haunted by this horrifying presence that seems to predict terrible disasters. The style is on point, and it's especially creepy because it's supposedly based on a true story.

On dreaditt, when I browsed there, it is generally considered either midsommar or hereditary. Those movies will make you feel levels of unease I don’t think many other horrors have ever achieved

Also I think Smile was unhinged and unusually scary. For a sci-fi horror annihilation was extremely good

Idiocracy.

Based on kid me who hasn't seen it again since, Fire In The Sky.

A more recent one, It Follows, not because it's some groundbreaking horror or amazing movie, but the dread of a force so unstoppable and inevitable that you can't get away from it no matter what you do is what makes it scary to me. I had dreams after watching it where something is chasing me but I can't remember even looking back or seeing it. And you know how running in dreams works right? It doesn't. So I had to do the grab the ground and pull yourself forward move to run away. Anyone who has tried running in their dream knows what I'm talking about.

Pan's Labyrinth

It’s rare for me to find a movie that actually scares me, but Sinister is one that really make me feel afraid. The soundtrack alone does it for me

Tension scares? Visual scares? Gore/ physical pain scares? Different stuff works for different folks.

Depending on what floats your boat, a few that come to mind would be Alien, Event Horizon and the Saw franchise.

Ringu - scares me every time I see it.

Many of David Lynch's films too, although none are out and out horror. They almost all have moments though - Mulholland Drive, Inland Empire, Fire Walk With Me, Blue Velvet. Really insidious moments that just won't leave you.

Blair Witch Project - many people would disagree, but I think it's a masterpiece of fear.

Twin Peaks when Bob climbs over the couch towards the camera. I was extremely uncomfortable like he was actually looking at me and was going climb through the TV screen.

Lake Mungo is a good combination of both

I love "The Strangers", it's the only movie I consider actually terrifying, and not just jump scares.

Creep was possibly the only movie to move the dial.

Smile and Barbarian were scary.