Surely not a movie for everyone; perhaps the best example of a sociological character study if I've ever seen one. The insight into a psychopath is quite unsettling yet unfiltered. On one level, at least mine, it's kind of fascinating, even though it's also exceptionally disturbing.
Taxi Driver (1976) - A mentally unstable Vietnam war veteran works as a night-time taxi driver in New York City where the perceived decadence and sleaze feeds his urge for violent action, attempting to save a preadolescent prostitute in the process.
The Fifth Element (1997) In the future, a cab driver unwittingly becomes the central figure in the search for a legendary cosmic weapon to keep Evil and Mr Zorg at bay. Bruce Willis, Milla Jovovich, Gary Oldman