It is very difficult to choose just one Spanish film. It depends on the era, the genre and other factors. I can make a short list of Spanish films that managed to move me at different stages of my life:

Mar Adentro
The Lovers of the Polar Circle
The Girl with the Golden Panties
Amantes
Atame
The Beehive
Open Your Eyes
The 507th Box
The Ages of Lulu
The Kingdom
Magical Girl
No Peace for the Wicked