If you liked the long version of Dumplings then you might want to check the three extemes release containing the short version plus two others - can't remember what the DVD is called in the UK as they messed with the numbering. Oldboy is part of a theme-based series, so you could check the other two (although they aren't as mainstream).
Here's a random list of good films (IMO) from different genres, although mostly horror: Tale of Two Sisters, Taegukgi (aka Brotherhood), Dark Water, the Ring films, My Sassy Girl, Il Mare, Battle Royale, Audition, Shutter, Quiet Family, Infernal Affairs series, the Ju-on films, etc... Just make sure to avoid the remakes, which often have the same names.