It might seem like a bit of a cheat to bundle the original Chinese movie with the American remake, but in fact they are both fine versions.
When I saw Infernal Affairs, I thought it was a fantastic concept. Two young police officers start their careers. One is sent undercover into the world of organised crime and the other is on the payroll of those same criminals, essentially undercover as a cop.
The way the story plays out and the terrific resolution once they finally meet, makes this one of the best cop movies out there. Both versions show a fairly honest (I think) representation of violence, loyalty and conflict.
Aside from the core story, another interesting aspect to was the different ‘cultural’ endings. The Chinese version had two endings – the mainland unsurprisingly got an ending where justice prevails – but Hong Kong got a more ambiguous ending. The American version ends with, unsurprisingly, a bit of vigilante justice.
Personally I like the original better but the remake has a stellar cast. Take your pick.