Although I am not a DeNiro fan, "Taxi Driver" remains my favorite movie of all time. NOTHING I have seen before or since captures the feelings of alienation that a young man has, particularly in an urban environment. Like much of 50s Noir, the city is as much a character as the, well, human characters. Besides, its still cool to see the old Belnore Automat (where my grandfather used to take my mother), as well as the East Village and Times Square in the Beame years.
The coffee shop on 14th street and Katz' deli on Houston!
The movie just never clicked with me, although I do like all of the actors in it.
For the alienation of a young man feeling, you should check out the book A Fan's Notes by exley.