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.