I know this is very broad. The way I see it, it's very difficult to name one particular film to be #1, but if I were to name my favorite film it would have to Kubrick's Dr. Strangelove. I find the dark comedy, setting, and writing to be some of the greatest work ever completed by man. And the star-studded cast that includes Peter Sellers (triple casted), George C. Scott, and Slim Pickens doesn't hurt either. Others of mine that are contenders would be Evil Dead II, Pulp Ficiton, Fargo, Punch-Drunk Love (or anything P.T. Anderson), A Serious Man, and Dawn of the Dead (the original), along with so many others.
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For those asking, I consider films to be much more formal, have a message, and require serious thinking to understand what's going on or behind the scenes. Whereas movies are easily approachable and can be enjoyed by anyone and don't necessarily have a serious message to give. A more recent example of a film would be District 9, and a more recent example of a movie would be the Hangover.
[Edit]
For those asking, I consider films to be much more formal, have a message, and require serious thinking to understand what's going on or behind the scenes. Whereas movies are easily approachable and can be enjoyed by anyone and don't necessarily have a serious message to give. A more recent example of a film would be District 9, and a more recent example of a movie would be the Hangover.