It goes without saying that David Lean's classic, Lawrence of Arabia, is my favorite film of all time. I first discovered the movie when I was 15 and became obsessed with it, renting it weekend after weekend and it was the first DVD I ever bought.
I've always wanted to put together a list of my favorite films but I know for certain that at the very top is Lawrence and I know a lot of filmmakers who would do so as well.
Steven Spielberg has spoken about the influence it had on him when he first saw it as a teenager and how he goes back and revisits it before starting his next film. Another great filmmaker who recognizes it's brilliance is Martin Scorsese.