Impossible to answer for me. Sure we have the recognised classics from the 70's (No Taxi Driver Loathsome? shocking!), the 80's gave us the age of the blockbuster and video nasties the 90's a return to more character based films and the ago of CGI but the 20's gave us cinematic language, the thirties ushered in sound, the 40's gave us Film Noir, the 50's gave us paranoia and Sci Fi, the 60's gave us kitchen sink drama and the emergence of the auteur and the 00's onwards gave us a right load of old crap
If really pushed I would say that the 70's really was a great time for movies with theatre fillers that possessed intelligence and boundary pushing films in terms of taste and decency where even mainstream directors were given more free reign to fulfill their ambitions (for better or worse, usually the latter - Heavens Gate New York, New York etc) but the 80's for anyone over the age of 30 is much more defining in terms of cinematic appreciation due to VHS.
Members here such as myself, bob, Janszoon etc were exposed to so many films due to VHS but also had to sit through an insane amount of crap due to the lax laws (at that time) that we can happily watch a B movie and appreciate it's merits as much as a bona fide classic, because we were not always fed a diet of well produced/financed films and so the expectation levels are lower and much the better for it. *
I have said on here many times that I would much rather watch a film that cost $3 million where a filmmaker has to use his imagination and challenge himself than a $30 million film that looks like every other film.
I waffled a little in this post didn't I? Sorry!
*Apologies Jans and bob if it isn't so but I know it is!