There must be one year that is objectively the best. Just like there must be one movie that is objectively the best.
How can one possibly tell? Probably impossible, but that doesn't mean there's no answer. But hypothetically if there was some simulation machine that allowed us to expose every movie ever made to every person and that machine could analyze the person's brain while watching to determine the emotional reaction and long term impact and rank every movie accordingly to which made the largest impression. And then we average everyone's lists and an answer would pop out of the greatest movie ever made.
The answer would probably be something kind of lame like Avatar or some Pixar movie because of it's wide appeal.
Maybe you could argue that most people aren't educated enough to appreciate the finer details of more sophisticated movies so then you could add to the simulation a parameter that makes it so everyone learns everything about movies and film criticism like how neo learns kung fu, and then you run the program again.
Finally you would get the correct answer.