Long Arm of the Law II (1987)

A brutal, gritty, no holds barred tale of three criminals recruited to go undercover and break up a criminal ring. The film is melodramatic and confusing at times but makes up for it with earnest portrayals and a hardcore, non-flinching approach to gang violence and the people on both sides of the law who use increasing amounts of violence to get the job done. It also shows loyalty and betrayals on both side without really choosing sides or glorifying the violence shown. Elvis Tsui is definitely an underrated actor. There is an abundance of raw action, stunts and violence with just a smidgen of Martial Arts, so if you like gritty no bullshit movies and/or heroic bloodshed films look no further.
After seeing this one I'm definitely tracking down the first and third stand alone films in this series.