Its impossible to pick just one but i did.

Its my favorite because just like all my other favorites, It is so believable. The movie sucks me in and i enjoy watching it every single time.

Other favorites:
The Deer Hunter
On the Water Front
Cool Hand Luke
Plains Trains Automobiles
The Fugitive Kind
Scare Face
Carlitos Way
Rebal Without a Cause
Saturday Night Fever
Forest Gump
Hud
The Good The Bad and The Ugly
Dirty Harry
Apocalypse Now
Goodfellas
My Cousin Vinny
Rocky
First Blood