All I knew about The Matrix was it had karate, and the slow mo looked cool. I walked out and said "I have no idea what I just watched, but it was amazing."

Dog Soldiers

Completely blind? Something with a lot of descriptive dialogue and good sounds and music. 😏

Enter the Void. Best enjoyed on acid with friends 😉

Shutter Island. It is both a movie that is better when you don't know what it's about and when you've already seen it once. Watch it twice.

Audition

The Endless

Two brothers go on a road trip to investigate the UFO cult they spent time in during their childhood. I'll leave it at that to avoid spoilers

I posted this here a couple of months ago: Old Henry (2021). It's one of the best slow-burn character dramas to come out recently, in my opinion. I would steer clear of any synopsis of this movie, go in blind and enjoy the story.

The Empire Strikes Back

The perfect host is pretty good!

"Arlington Road" and "Cabin in the Woods"

Lots of great suggestions below already, thought I'd add The Prestige. Peak Nolan in my opinion, and definitely benefits from going in blind.

I've got 2 Id recommend: Matchstick Men Identity

I'm gonna try for some where there's a chance you don't already know what they are:

Bad Boy Bubby

El Secreto de Sus Ojos

The Music of Chance

I'm Not Scared

I feel like you need to have a pretty good sense of someone’s tastes to make a “trust me”-style recommendation like this.

You do. Also, if you ask specifically for movies you have to be unprepared for, won't that make you prepared for them?

Snowtown,but really dont trust me.