As a film buff I find the prospect of having one favourite film of all time laughable. Perhaps its because of the sheer volume of movies i've seen, but i'll give a few recomendations of the top five movies i've seen in the past few months.
Ben Hur - Its four hours long, but Charlton Heston makes it all worth while.
Rashomon - Akira Kurosawa's groundbreaking film about reality, and humanity. This movie is a masterpiece.
Alien - Its fucking Alien.
The Seventh Seal - Ingmar Bergman's favourite subject is death and this film deals with it by pitting a man against the physical manifestation of Death in a game of chess.
I'm a Cyborg, But That's Okay - Chan Wook-Park's tenth film I believe. While not a masterpiece this film is odd and crazy in the best possible way, after all its about a crazy woman that believes she is a cyborg and a man who thinks he can steal souls.