The French Connection
Merveillouse. A classic for good reasons.
Popeye, a lowly but fierce narcotics cop, stumbles into a big time heroin sale between some lowly Italians and a too clever Frenchman. The chief lets him and his partner loose, but with some Federal babysitters for the wiretaps and stuff.
Frog 1, as he is named, gives the cops the run around, toying particularly with our protagonist.
The police gets most of the gang, including the evidence, but the epilogue shows most of the involved get away almost scot free, particularly Frog 1.
Between that, there’s a bunch of iconic and high octane chase sequences that put modern movies to shame. Incroyable!