Mirror
The Circus (Chaplin)
Early Summer
Nights of Cabiria
Flowers of St. Francis
The Third Man
Passion of Joan of Arc
Winter Light
Young Mr. Lincoln
Dumbo
In the Mood For Love
Aguirre, the Wrath of God

I wouldn't touch a frame on any of these films.

There are a lot of great films that seem as if they could improve from some slight alteration but aren't really any less great because of it. I think a desire for perfection can actually hamper a lot of artists from tackling the stuff that is actually more important. Pasolini films are less perfect than, say, Wes Anderson films, but they are much greater works of art.