There is no uniform opinion as to which film is the greatest ever, but a number of films have been presented as such.
Generally speaking, these tend to be films that can be said to have advanced both the art form and the technique of cinema, and are usually agreed upon by a consensus of critics and scholars to have been inordinately successful on both these grounds.
Different groups have very different criteria for what is greatness, however. While critics and film aficionados would agree to the above definition, others would argue that popular appeal is a more important measure of greatness. Those who work in the movie industry, however, would probably consider the most profitable film to be the greatest.
There is very often incompatibility between these definitions. Commercial popularity is rarely a criterion for artistic excellence; often the films most popular with the public are entertainments that work within established artistic formulas, rather than striking out into new artistic territory. And yet it can fairly be said that the films considered the best of all time are, for the most part, quite popular with general audiences as well.
Films that are considered among the greatest in their particular genre:
Films that are considered greatest from a particular country:
Movies that are widely considered important, if not the greatest ever
See also: List of movies that have been considered the worst ever
Lists of Great Films