As far as art forms go, cinema is fairly recent. Music has been around for thousands of years, as have sculpting, painting, writing, and poetry. Film has only been around since the very tail end of the 19th century.
The earliest surviving movie is Roundhay Garden Scene, a 1.6-second silent film from 1888.
Taking that into account, it's almost unreal how far the craft has come. In just under 150 years, movies have gone from silent, blink-and-you'll-miss-it shorts to dramatic masterworks like The Godfather and CGI extravaganzas like Avengers: Endgame. The disparity between then and now is hard to wrap your head around, and that's to be expected. As the art form evolves and new techniques are developed, it's only natural that the end product gets better and better.
In the last century, we've gotten exquisite pieces of filmmaking. In addition to the aforementioned Godfather and Avengers films, we've seen devastating war epics like Apocalypse Now and 1917; world-class superhero flicks like Logan and The Dark Knight; haunting pieces of horror perfection like The Exorcist and Hereditary; and many, many others.
What decades stand out as having the best? That's very, very subjective, but let's have a look.