Une Partie de Campagne (Renoir) is the best film of under an hour.

Some great animated shorts:

Street of Crocodiles (Quay bros)
Tale of Tales (Norstein)
The Wrong Trousers (Park)
The Mascot (Starewicz)
What's Opera, Doc? (Jones)
Duck Amuck (Jones)
Begone Dull Care (Mclaren) handpainted
Colour Box (Lye) handpainted

Live action (many are experimental films):

A Trip to the Moon (Melies)
Voyage beyond the Impossible (Melies) silent
Dream of a Rarebit Fiend (Porter) silent
Un Chien Andalou (Bunuel) silent
Entr'acte (Clair) silent
Twilight of a Woman's Soul (Bauer) silent
Daydreams (Bauer) silent
Cops (Cline) silent
Mysteries of the Castle of Dice (Man Ray)
The Music Box (Parrott)
White Mane (Lamorisse)
The Red Balloon (Lamorisse)
Meshes of the Afternoon (Deren)
La Jetee (Marker)
Birth of a Robot (Lye)
Neighbours (Mclaren)- pixillation
Inauguraton of the Pleasure Dome (Anger)
Denko (Camara)

one i'd love to see:
Isle of Flowers (Furtado)