Production values for film reached a peak in 1953 with "The Robe" and the introduction of Cinemascope. Photographic clarity, color (original Technicolor), sets-- the overall photographic look-- had never been better.
From this point through 1963, a time of low inflation, filmmakers had the money to spend on lavish productions. The result was a series of sumptious flicks that were like a series of breathtaking paintings.
As well, the quality of screenwriting remained very high. Many of cinema's greatest directors, such as Alfred Hitchcock, David Lean, John Ford, and George Stevens, were at their peak. Best of all was the movie music of the period. Music is an underrated element of the art of film-- crucial to the highest artistic experience. From 1958 through 1963, in a conjunction of peaking talent, many of the best ever film composers did their greatest work: Miklos Rozsa; Bernard Herrmann; Dimitri Tiomkin; Elmer Bernstein; Jerome Moross; Maurice Jarre; Alex North; and likely a few I'm forgetting.
If film is a combination of artistic elements, especially the visual and the aural, with narrative and character depth added, it's hard to choose a period when the elements better came together. They came together best of all in "El Cid."