Impact in what sense?
Financial, cultural, etc?
Financial has to be Titanic. First movie to gross a billion dollars.
Culturally is a more difficult question to answer.
i'd say wizard of oz for the sheer fact that more ppl have probably seen that movie than any other movie.
Hard to argue with that listCitizen Kane
Gone With the Wind
Psycho
Star Wars
2001
The Matrix
Jaws
The Blair Witch Project
Casablanca
And another movie I don't even want to acknowledge.