The French Connection
A bit like Bullitt, I feel as though this is mostly known for its car chase, and that's really unfair because it's a cracking movie all-round. Mind you, it is a great chase. Car vs. elevated train was such a genius idea.
Also like Bullitt, I get the impression this was way ahead of its time stylistically. It's got a very modern, almost documentary-like feel to it. Hackman and Schieder are excellent together. Love the ambiguous ending too.