"The Night Porter," is so dirty in a way no porno can be; it makes you want to take a long, hot shower or take a long, random drive after you watch it.
"400 Blows" should be there, of course, as well as "Contempt," with Bridgette Bardot when she was still hot. I'd add, "Discreet Charm of the Bourgeoisie" but it's probably been mentioned and I've never been able to finish it at one go (no subliminal pun intended).

I never did watch a Tati film but it's a new one to add to the Netflix queue.

I should add Pasolini's Salo or 120 Days of Sodom. A few college friends and I checked it out from a mom and pop video place back in the 1980s. I remember the proprietress giving us a weird look.

Also, for the so inclined, a Czech classic, Marketa Lazarova. Weirdly mind-bending. It's probably best to watch it during an acid trip.