I voted "impact on film," insofar as I believe that reference and allusion are really important, and films that affect other films are the most important films. As y'all know, I teach a class on The Wire, and I spend a whole class (late in S3) on the influences of revisionist Westerns and the first two Godfather movies on III.11, Middle Ground.
That said, that's not my answer. My answer isn't in the poll.
My answer would be "aggregated critical opinion, from a significant remove."
I place a lot of weight on expert opinion. Maybe a lot of people thought John Carter of Mars was a great movie. It certainly made some bank. I think it was the worst movie I've ever sat through the entirety of. But there is a long history of the experts messing things up in the short term and getting them right in the medium or long term. Ebert made his rep on being the only guy speaking up for Bonnie and Clyde at the time, when he was not really anybody. The Victorians thought several of the "Big Six" Romantic poets sucked. Bach was lost until Mendelssohn re-discovered him.
But in the long run, the people who spend their lives studying something really seriously get stuff right. Christopher Frayling knows more about the revisionist western than almost anyone. I value his opinion more than any of y'all's, sorry. I care most about the aggregate opinion of the people who study whatever the subject is most carefully, maybe adjusted for a few years remove. People were so-so on Vertigo at first; now you would be hard pressed to find a real film critic who thinks it's lousy.
Treating the list in the newspaper of the current bank on the movies in theaters this week might be the worst possible way I can think of to assess quality of those films. We might as well assign values to the letters the titles start with and value it that way.
A movie is not about what it's about; it's about how it's about it.
---Roger Ebert
Some questions cannot be answered
Who’s gonna bury who
We need a love like Johnny, Johnny and June
---Over the Rhine