It would have had to have been a decade that was free to experiment in ways other decades weren't. This is either due to the MPPC / Hays code (1930-1968) or the rise of digital filmmakers and indy film.
So it's either the 1920s, the 1970s or the 1990s.
The 1920s because some very ingenius filmmakers were really discovering and pushing the boundaries of what film could do and ways stories could be told.
The 1970s because all the pent-up creativity and darkness burst into cinema after the demise of the Motion Picture Production Code, leaving us with some of the most memorable and powerful films ever created. There were also some free-wheeling, hippie filmmakers who managed to get movies made outside of Hollywood, leading to a significant indy, cult and exploitation explosion.
The 1990s because people who would never have been able to have a voice in traditional Hollywood were suddenly able to break into the industry and tell amazing stories. The rise of digital technology and the explosion of the indy film industry (in large part due to Harvey Weinstein) .
I'm expecting another modern digital filmmaking revolution (because now $5000 cameras can perform like $100000 cameras and everyone and their grandmother can do CGI effects), but it didn't seem to happen in the 2010s. (Probably due to blockbusters and reboots smothering low budget fare as the industry radically changed.)