Firstly I've been a huge fan of Scorcese since I can remember being a fan of anything , he was probably the first director that a really got into. Well I can't say what his "best" film is but I can tell you my favorite of all ; Taxi Driver.
I first saw Taxi Driver when I was about 9 or so, I'm 26 years old now. I remember watching the film then very well, and I remember wasn't able to understand a lot of the themes of the movie. One thing I did remember was the final shoot out scene. Seeing that as a kid, quite honestly changed my life. Sounds dumb, but I think that scene de-sensitized me towards movie violence -- and mildly traumtized me.
As said before, I saw this movie when I was about 9 or so and I probably watch it at least 1 once a year . Watching Taxi Driver now, I look at it in a different light, basically I've grown up with it. Still everytime I watch it, it holds my attention it has everything you can want in a movie really. I really can't say that about many other movie, this movie is nearly perfect to me. Taxi Driver to me is not only Scorsese's best, but one of the best films of all time-- to me of course.
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BTW: Yes, I had a lot of time on my hands today.