Taxi Driver was arguable Robert Di Niro's best movie (some might argue the Godfather but he wasn't the main protagonist in that one) and kind of mind blowing for 1976. It helped launch his career into the acting legend he is today and won four Academy Awards including Best Picture. It is considered one of the greatest films of all time and if you have not seen it you should immediately.
It is of course the story of Travis Bickle, a lonely obsessive living in New York City who becomes disgusted with the filth he sees around him every day. He falls in love with a campaign worker but messes that up and loads up with illegal handguns. Eventually he uses them to rescue an underage prostitute named Iris from her human trafficking pimps in an epic gun battle all the while considering killing a local politician. Truly an innovative movie and given the current political climes very ahead of it's time.