Thought to add some comments on my list, since the others all posted so many films. 
I have seen some 300 films from the 50s, but if you look at my list , it's very limited. I'm not trying to be a snob, but sadly I didn't see many masterpieces in that period.. 
There are very good films I enjoyed much and though I value their importance they just didn't make it. 

Some examples would be:
Orphe (Cocteau)
Un chant d'amour (Genet)
An American in Paris (Minelli)
A Streetcar named Desire (Kazan)
Singin in the Rain (Donen, Kelly)
From Here to Eternity (Fred Zinnemann)
Roman Holiday (Wyler)
Seven Samurai (Kurosawa)
Rear Window (Hitchcock)
Ordet (Dreyer)
Killer's Kiss (Kubrick)
To catch a thief (Hitchcock)
The Searchers (Ford)
Forbidden Planet (Fred Wilcox)
The Killing (Kubrick)
King Creole (Curtiz)
Hiroshima mon Amour (Resnais)
Ghost Story of Yotsuja (Nakagawa)
The Bridge (Bernhard Wicki)
Floating Weeds (Ozu)

I didn't list them as possible additions, because my memory of them is still very intact. Of course they could still make it on the list with another viewing, but they will have more chances in the farther future, when my taste in films will have changed a bit more.