Why is Citizen Kane considered the Best Film?
The American Film Institute and the British Film Institute have Citizen Kane listed as the number one movie of all time.  The British Film Institute has had this rating of Citizen Kane for 50 years.  There are multitudes of reasons why this is a deserving title with a few examples below.

1. Citizen Kane was a thinly veiled movie about a powerful person.  Even before the movie was released newspaper magnate William R Hearst became aware that Kane had many similarities to Hearst.  Both controlled media empires, ran for office, and had giant mansions filled with odds and ends.  Where it allegedly became personal was when scenes showing Kane's wife put through an humiliating run in showbiz propped up by Kane's largesse mirrored Hearst a bit too closely to real life.  Hearst reportedly offered to buy all copies of the film and told newspapers to not print anything or even mention its name.

2. It heralded a new age of cinematography in 1941.  Citizen Kane pioneered in the mainstream many types of compositions and angles we now find quite common.  For a low angle shot in an office to show Kane's towering presence film cameras were too heavy and bulky to sit on the floor.  Cinematographer Gregg Toland solved this by having a hole dug deep enough for camera and tripod to achieve the desired effect.  Also the concept of "deep focus" or achieving a large depth of field to make characters near and far appear sharp is difficult to achieve.  These scenes were used to great effect to portray Kane as being surrounded and alone with his wealth later in the movie.  They also provided a literal window into a young innocent Kane during a tense scene inside the house, to which Kane was blissfully unaware but the audience experienced both to jarring effect.

3. Orson Welles was famous and an excellent promoter.  Up to this point Welles most famous exploit had been his radio re-enactment of H.G. Wells "War of the Worlds."  Choosing to follow a newsreel style format and not having an intro saying this is a work of fiction many listeners at home were swept up in the drama and thought an alien invasion was happening.  His popularity from this event carried a great deal of interest over to his first film.  For his first film to come together with a compelling story, technical wizardry, and cultural relevancy truly made it the standard for what a "Best Film..." should be.

As a sidenote in 1921 Citizen Kane won 9 Academy awards including Best Picture, Director, Editing, and Cinematography. Orson Welles would never have a similar level of success with his later films.