My favorite movie

McDowell and Kubrick chatting and being awesome. 

This is a very hard question. I can't say I have an absolute favorite movie, but I think the movie that changed my perception of cinematography is A Clockwork Orange. This movie absolutely blows my mind the first time I saw it, because I was used to more "mainstream" action movies with immortal protagonists saving the world from terrorism. This movie shows precisely the opposite part of the story: 

The main character is called Alex DeLarge, a fourteen years old teeneger played by (the always awesome) Malcolm McDowell. Alex lives in a disthopian future where London (and presumably other cities in the world) are infested by numerous gangs. Of course, our main character is the leader of one of those gangs, and the movie focuses in the "misadventures" of this gang in the first act. 

The breakpoint of the story comes when Alex is sentenced to 14 years in prison after murdering an old woman. After two years in prison, Alex offers himself as a guinea pig for a new aversion therapy recently developed named "Ludovico technique", whose researchers claim it can rehab a criminal in two weeks. 

My favorite part of this film is the third act, when the viewers can see the complete "rehab" process of Alex with the Ludovico Technique and its effects. Here the film also shows the consequences of Alex previous crimes, and how this events change the life of his victims. 

 The reason why I like this movie is primarily the setup and art direction. The music and aesthetics of the film are masterfully put together in almost every scene, and also works as a metaphor of how Alex sees his criminal life, more like a "piece of art" than mindless destruction. I also like the acting (specially McDowell), but with a script done by Stanley Kubrick is really unlikely  that the cast could fail in their portrayal. 

Since A clockwork orange is a movie based on a novel, I think this is one of the few times that the film surpasses the original source. The only aspect I kind of miss from the novel is the ending, mostly because I think it is more realistic and do a better job showing that humans aren't inherently evil. This is one of the reasons why Anthony Burgess (the author of the original novel) despises the Kubrick adaptation.

As I said earlier I can't say this is my favorite movie at the time, but the influence in my perception of cinema as a form of art is immense, and I watch this movie almost every time I catch it on cable tv....

(I just realise that I did a post in 2010 about the same movie! OMG! Well, that shows how important is this movie for me ;D.)