4. Oldboy
From what I hear, the other two films in Chan-Wook Park's Vengeance Trilogy - Sympathy for Mr. Vengeance and Lady Vengeance - are pretty upsetting, but Oldboy is the only one that I've seen so it's the only one I can talk about. Oldboy is the story of Dae-su, a businessman that is kidnapped on his daughter's birthday and imprisoned for 15 years for seemingly no reason. After exactly 15 years, he is released without explanation and begins a quest towards vengeance. In search of his captors, Dae-su falls in love with a young chef named Mi-do. Eventually, Dae-su confronts his captor and discovers the secret to his confinement. That's when things get gut-wrenching. Dae-su's captor, Woo-jin, orchestrated this elaborate plan as payback for Dae-su's action when the two were at school together. Dae-su witnessed Woo-jin in an incestuous relationship with his sister, and proceeded to spread the rumor to the school. The rumor drove Woo-jin's sister to suicide. But it wasn't enough for Woo-jin to kidnap Dae-su. He took things one step further: Mi-do, the chef that Dae-su fell in love with and made love to, was actually Dae-su's daughter, all grown up. He was tricked into having sex with his own daughter. Now go take a shower.