Some of the most original and best movies I've ever seen came from the 70s, so I'll go with that decade.
Both Godfathers, Clockwork Orange, One flew over the Cuckoo's nest, Taxi Driver, Chinatown, Exorcist, Mean streets, Dogs Day Afternoon, Deliverance, Deer Hunter, Rocky, Alien, Star Wars, Jaws, Close Encounters, Halloween, Watership Down, Animal House, Enter the fucking Dragon!
This shouldn't even be close. Most of the movies which literally created clones or blew up entire genres started in the 70s.
The 80s just pretty much remade shit from the 70s, lol, and I grew up during the 80s and 90s.