As a huge film buff going way back, picking just one is completely impossible. With so many genres and so many styles throughout the years, I would have to list at least 30 or so before I could say I liked any one any more than the others. The simplest thing I could do would be to just link to my IMDB list but I haven't updated it in so long there's probably a few years of missing films.
Anyway, since this is no longer a "give-away" thread for listing just one, I'm gonna go ahead and list an actual list because they are honestly all in a deadlock for number one...
The Treasure of the Sierra Madre
The Shawshank Redemption
Forbidden Planet
The Ten Commandments
Blade Runner
Pulp Fiction
The Shining
Key Largo
The Caine Mutiny
It's a Mad, Mad, Mad, Mad World
Monty Python and the Holy Grail
Dead Men Don't Wear Plaid
Dances with Wolves
The Mosquito Coast
The Long, Long Trailer
Mulholland Drive
The 10th Kingdom
Ed Wood
Big Fish
Amelie
The City of Lost Children
The Fifth Element
Rear Window
At Play in the Fields of the Lord
What Dreams May Come
The Breakfast Club
Planes, Trains and Automobiles
The African Queen
The Empire Strikes Back
The Love Bug
Jurassic Park
The Abyss
Aliens
Dark City
Stripes
Young Frankenstein
Ghost in the Shell
Finding Nemo
Casablanca
Willy Wonka and the Chocolate Factory
I know I'm forgetting quite a few...