Enter the Dragon
1973
http://en.wikipedia.org/wiki/Enter_the_dragon
It has everything but a hot-rod Duster throwing gravel around a parking lot of bikers, but I can forgive that one shortcoming. 
I dare you to come up with something better.
Enter the Dragon is great, but come on. The Godfather left a horse’s head in its bed.
Star Wars
The Outlaw Josey Wales
It’s so obvious I don’t know how the question can be asked.
Unless you include porn. Then it’s a toss up between* Deep Throat*, The Devil in Miss Jones, and* Behind the Green Door*.
Godfather
Godfather 2
A Clockwork Orange
The Sting
Dog Day Afternoon
Chinatown
Jaws
Taxi Driver
One Flew Over the Cuckoo’s Nest
The Man Who Would Be King
Young Frankenstein
Star Wars
The French Connection
Apocalypse Now
The Deer Hunter
Rocky
Annie Hall
Close Encounters of the Third Kind
Network
The Conversation
For starters. Enter The Dragon is a fine film, but… seriously?
I’ll agree with Anamorphic’s list, but it left out “Breaking Away” (1979)
The only problem with Star Wars is that is produced three profoundly awful movies that essentially ruined the whole franchise–until Lucas dies and good remakes can be produced.
Yes, seriously. And you can only pick one.
I never understood the mindset that the prequels ruined the originals. I can still watch them with as much enjoyment as ever, and I just pretend that the PT does not exist.
Oh wow, actually one of my favorite films of all time! I am ashamed I left it out; thank you for the addition!
Alright, then The Godfather.
Star Wars
or
Monty Python and the Holy Grail.
The seventies were the golden age of film making in my book, and I agree to what Anamorphic is suggesting (?), the list of masterpieces is so long and diverse one can’t just pick one movie out of it. A Top Ten would be more reasonable.
Excellent point! If just one, though, The Godfather.
I’m saddened that I have to be the first to mention the greatest movie of the 70s, Jeremiah Johnson.
What, no love for Zardoz? 
The Texas Chainsaw Massacre.
Could we get a ruling on the porn issue?
Star Wars certainly had a bigger impact on the art as a whole. Enter the Dragon had a lot of impact on martial arts movies. But Star Wars had impact on movies in general.