I can't say my FAVOURITE movie, but in my top i have : old boy, clockwork orange, trainspotting, drive, fight club, requiem for a dream and much more...
When you think about it, it's the movies you watch the most over the years that can be considered your favorite. I would say the first Star Wars (1977) and Planet of the Apes (1967) are the two movies I watched the most and this is without including the traditional Easter or Holiday movies I watch every years.