I Saw the Devil was really good. But both Confession of Murder and Oldboy was better than I Saw the Devil IMO. Lady Vengeance and Joint Security Area from the Oldboy director were also really good.

If you like martial arts you should try Man of Tai Chi (for the story/themes) SPL trilogy because it's so damn good and Raid and The Raid 2: Berandal because the following scene was actually cut out of the movie to make it a better movie.