for me, it's oldboy (2003). honestly idk if i could tell you exactly why i love this movie quite as much as i do, but man is it not something i ever tell other people that i love lol

Before Sunrise and its two sequels. I love them to death but they're a LOT of talking so I know they're not for everyone.

And Grave of the Fireflies because, well, if you know you know.

Possession (1981), a very chilling horror movie that was apparently banned in theaters when it was first released. I honestly love it and the plot and acting are phenomenal, but I never tell ppl about it, even if they tell me they're a horror fan