It's cheating as it's not a film, but 'Braquo' is teeming with corrupt cops and is (for me) up there with The Wire.
In fact the French do the corrupt cop genre remarkably well (Engrenages, 36, La Balance, Dobermann, Mesrine - all feature some slightly dubious application of modern policing techniques).
But Braquo really is next level and should be seen by as many people as possible, not least of which because it introduces one of the all-time great villains. You have been warned.
If you will suck my soul, I will lick your funky emotions.