While Park Chan Wook's "Vengeance Trilogy" is quite the twisted treat as a whole, a lot of that cult status comes from the trilogy's middle film, Oldboy. All three films center on revenge stories, but Oldboy's is so unique, but also bizarre, that it went on to win the Grand Prix at the 2004 Cannes Film Festival. Yes, the movie with the "hammer scene" also won some pretty prestigious awards…and that's saying something.
We've already gushed at length about Oldboy in the context of comparing it to Spike Lee's remake, but in the "Vengeance Trilogy" it's easily the best. In fact, it may even be the best film in Wook Park's entire filmography.