This'll be quite a list... but I'll try... as you said -movies- I take more as one are granted...
In no particular order...
American History X
Smokin' Aces
Snatch
1984
Stalker
21
Che (both movies)
City of God
Hotel Ruanda
12 Monkeys
Trainspotting
Acid House
Fear and Loathing
La Haine
Reservoir Dogs
Boondock Saints
...
//Edit: And if we count Anime too...
Jin Roh
Ghost in the Shell
There's a lot more... listing them all is way too much. Lots of good movies out there tbh.
If I got to choose one of the listed above, I'd go with La Haine.
Grave of the Fireflies
Tron
Tron: Legacy
Pulp Fiction
Gladiator
Ghost in the Shell: Innocence
Cowboy Bebop: The Movie
Goodfellas
but the list goes on to many other...but if you ask just one, the for me the most important piece of cinamatography is Grave of the Fireflies (of couse the animated original)
Nothing as an absolute favorites.
But I'm a bit of a sucker for animated stuff.
I'm also not certain if I should be ashamed of mentioning recent watches.
How to train your dragon.
Rango.
But one of them as a favorite movie(?) of all time, could be monthy python and the holy grail. I guess?
Controversially the worst script for a movie I ever witnessed may or may not be "the happening"
It simply doesn't make sense.
"Let's see how many ways we can 'coincidentally' make people mass suicide"?