morolf: I also enjoyed Infernal affairs (2002) and its 2003 prequel Infernal affairs 2 (the 2nd one in some ways even more so). It was remade in the US as 
The Departed, but I've never watched that remake, the original is probably superior anyway. 
 Both movies were directed by Andrew Lau and Alan Mak. 
  The originals are 
far superior; the acting, the writing ... everything is much better. 
 The US version was forgettable, I saw it when it came out and have never wanted to see it again.  
 Have you seen the 
Overheard movies, also by Mak ? They are in the same vein as Internal Affairs, not as good as 
Affairs I'll admit but still very good, especially the first one.