It's gotta be any decade before the end of the classic era.
Why? Because during the heyday of the big studios the ratio of high-quality films to less-quality was unmatched.
The so-called "factory system" did in fact, excel at maintaining quality control standards over the industry.
Source? Easy. Listen to Scorcese speak on this topic; he delves into it in one of his documentaries.
Getting down to specific cases: one of my buddies is practically an expert on the 1930s. He argues that the '30s were the best. The most stylish era; the period with the most auteurs. The glories of pre-code cinema. The heyday of slapstick comedy.
And then I had another crony once, who insisted that the 1950s were supreme. He points out how –in the '50s –the studios were at their peak of prosperity. Each of the studio genres were exploding with hundreds of hit films year after year. Indeed there were an astounding number/variety of successes in musicals, SF, fantasy, war, western, pirate, romance, drama. The numbers don't lie.
So: both of these blokes make very strong arguments.
What about the '40s? During this decade, the famous 'Hollywood style' of directing was perfected. But, on the other hand, Hollywood was simply overwhelmed by war propaganda.
The '60s? Screwed up by counter-culture and social change. Also the new pressures from television. Everything topsy-turvy.
The 1920s? Fabulous silent-film era; but this is still a period of 'infancy'. The nuts 'n bolts; the fundamentals were all still being formed. Film can't be considered a mature or evolved art form in those early days.
I myself? I lean towards the 1970s.
Reason?
In the '70s; big studios were still in the game but their contract system was finally unshackled. So you get plenty of studio flicks but also an explosion of independent filmmaking which was suddenly available.
You get indie filmmakers & revisionist filmmaking but –importantly –most of these talents, had matriculated with crucial studio training.
And for the first time, movies could display nudity and sexuality; in-depth human psychology. Handling these sensitive subjects was finally approached maturely. The variety of flicks is just staggering.
In general, the '70s were the decade when US film acquired a 'consciousness'; the power of self-reflection. Hollywood finally began to look at itself, revise itself, and begin to grow up.
Paul P. Powell, Pool Player

Thanks for proving me right again.
https://en.wikipedia.org/wiki/Race,_Evolution,_and_Behavior

I agree. 80s.
My password is password.

In the end, I think it would be the 1940s. While World War II took some top actors away from the movies for a few years (some like Leslie Howard permanently), still it gave some great topics and also a purpose to the movies.
Lots of great actors and actresses, great directors were working in the 1940s.
I think if I did my 100 favorite movies, about half of it would come from the 1940s.

None.

Hard to say anymore I have watched so many films in the past five years.
Before that I would have said the 80s for sure.
Now I am not so sure it's not the 70s.
Very close.

All good movies, but I wasn't being literal when I said ten movies. Just that other decades had far more movie gems.
Chopper: I'm just a bloody normal bloke. A normal bloke who likes a bit of torture

Dawn of the Dead
The Sentinel
Race with the Devil
Burnt Offerings
Invasion of the Body Snatchers
Suspiria
Halloween
Black Christmas
Texas Chain Saw Massacre
Jaws
There! I did it for you. And those are just horror films. There's also The Godfather I and II, Dog Day Afternoon, A Clockwork Orange, Rocky, Death Wish, Nashville, Taxi Driver, The Taking of Pelham One Two Three, Life of Brian, Pink Flamingos, Five Easy Pieces and Apocalypse Now.

I can't name ten good movies from the 70s.
Chopper: I'm just a bloody normal bloke. A normal bloke who likes a bit of torture

Seventies