For me it was Eraserhead.

Tenet - too much confusing and also the worst ending ever. The "epic" battle was totally confusing. I didnt even see any enemies???? Felt like a paintballgame or something :P

Fat Pizza

2003

THX 1138. I saw it when I was a kid, and studied it as a teen. The subtlety of everything, from the camera angles, to the low key acting is great, but it was waaaaaay ahead of its time and no one got it.

I had to watch ‘12 Monkeys’ twice to understand it

the first 2 Matrix movies were awesome. they are jammed packed with Easter eggs, different philosophies, computer humor etc. also helped create courses and game theory economics.

The fountain has to do with eternal life stories and mythologies all put together. you need some background first.

#17 After the scene where they try to arrest Trinity i was completely confused by what I just watched. They did a good job of explaining it later with the Morpheus Red Pill Blue Pill scene.

Gummo should be on this list. Really anything early Harmony Korine.