According to the American film institute (
http://www.infoplease.com/ipea/A0760906.html)
it will be the Godfather (1972) which is ranked #3 for best movies of all time.
Other contenders would be
Star Wars (1977)
One flew over the cuckoo's nest (1975)
Apocalypse now (1979)
Chinatown (1974)
Annie Hall (1977)
The godfather part II (1974)
A clockwork orange (1971)
M*A*S*H (1970)
Close encounters of the 3rd kind (1977)
Network (1977)
The French connection (1971)
American Graffiti (1973)
Rocky (1976)
The deer hunter (1978)
Patton (1970)
And probably one of my top picks for best 70's movie would be Jaws (1975)