Infernal affairs (2002)
"A story between a mole in the police department and an undercover cop. Their objectives are the same: to find out who is the mole, and who is the cop." (IMDB)
A bit of a surprise this one as it was a solid crime thriller which although being influenced by western films did succeed in keeping it's HK film identity.
The main reason I picked this up were Andy Lau and Anthony Wong who's acting I enjoyed but the whole cast are very good.
Remade as "The Departed".