This question originally appeared on Quora. 
Answer by Ken Miyamoto, Screenwriter
Without any hesitation or doubt, the best example in my opinion is Steven Spielberg.
In his career that spans four decades now, Spielberg has proven himself to be the most accomplished director of all time. While there are many names that will and should be discussed (Robert Altman, John Ford, Woody Allen, Alfred Hitchcock, Martin Scorsese, and many more), Spielberg has carved an amazingly successful career in many different genres of film. To me, the fact that he has succeeded in multiple genres is key to my appointing of him as the greatest of all time.
Take a look at his achievements in many genres and types of films:
The Blockbuster Event Films
Thought Provoking Science Fiction
And if you think about it, some examples in those two categories are interchangeable, because Spielberg always offers more than just blockbuster filmmaking.
Historically Based Dramas and Period Pieces
Comedies, Romance, and Dramedies
Animation
Spielberg won a Golden Globe for The Adventures of Tintin. He was neglected an Oscar nomination for it, which I believe was one of the worst exclusions in recent Oscar history, considering some of the films that were nominated in its place.
Then you add the films not featured above, such as Duel, the two Indiana Jones sequels (I don't speak of the fourth one), The Twilight Zone, Hook, Amistad, and War Horse.
No director has SUCCESSFULLY directed such a wide variety of films.
He has been nominated for Best Director Oscars six times and has won twice (Schindler's List and Saving Private Ryan).
Seven of the films he has directed have been nominated for Best Picture.
Spielberg's films have grossed over $4,099,094,612 domestically (Adjusted to inflation, it's about $8 billion!)
That an average of $151,818,319 per film.
His Worldwide grosses add up to over $8.8 billion ... and that is unadjusted and on top of his domestic numbers.
Spielberg co-created his own movie studio, Dreamworks, which still thrives today, and he is also one of Hollywood's most prolific and successful producers as well (Men in Black franchise, Back to the Future franchise, Gremlins, Transformers, Poltergeist, and a hundred more).
So I think there is no question that Steven Spielberg is the most accomplished director of all time.
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Answer by Mark Hughes, Screenwriter, Forbes Blogger
Does the "all time" aspect could mean we have to weigh the daring, difficulty, and intricacy etc within the context of different filmmaker's particular working periods? Or it could mean we have to weigh them all purely on side-by-side equating alone, with no accounting for changes to cinema or social conditions and so on? These points matter, I feel, because they could certainly change the answer I give.
I'm inclined to agree with Ken that Steven Spielberg is probably pound for pound the most accomplished overall director in history. I think even people who might not rank Spielberg as the "best" filmmaker of all time would probably say that if we measure all directors against one another without adjusting for things like the era and level of technology etc each had to work with.
Of course, we also have to consider that the question doesn't specify whether to weigh each filmmaker's accomplishments in the context of their own time, which might make a difference to some folks. For example, Fritz Lang's accomplishments were pretty phenomenal if you look at what he did and WHEN he did it. Can we really think of any contemporary filmmakers who accomplished some of the daring and amazingly complex things Lang did in his career during that time? He's phenomenal regardless, but with this added context you really get a truer sense of how much he accomplished, which can be hard to see from our position today.
Or how about considering the depth of daring and risk taken by filmmakers, compared against the quality of the outcome of those gambles? And weighing this time after time for filmmakers who made careers out of wildly divergent material crafted with great complexity and then produced with a level of intensity and difficulty hard to match by any other directors? If this were our guiding criteria, I feel Werner Herzog would easily rise to the top. He is, as some know (to much controversy), my personal choice for "single greatest living director," which is not necessarily at all the same as "most accomplished," but the reasons for my selection of him on that other question might inform one perspective as to how to judge the "most accomplished" director.
Filmmakers who have had a major impact on the history of cinema and inspired future generations, who have been mimicked and referenced in other filmmaker's work, and who have a solid film resume could be another excellent criteria for answering. That, then, would suggest names like Jean-Luc Godard, Federico Fellini, and Alfred Hitchcock should be top contenders. Even some more modern names -- like Quentin Tarantino -- should be high on the list using these considerations.
What about filmmakers who have a specific period of work that really defined an era of filmmaking and are among the greatest films ever created, and which all had lasting impacts on cinema and inspired the future of entire genres while also inspiring generations of directors to come? Francis Ford Coppola and Stanley Kubrick both stand tall here, and of course the question's criteria about daring and intricacy also apply. These two directors made four of the greatest Hollywood films of all time -- The Godfather, The Godfather, Part II, Apocalypse Now, and 2001: A Space Odyssey. There is a level of brilliance, of complexity, of reflecting everything pure and true about cinema, in each of those films, and their impact and importance is just one example of the accomplishments of these directors.
But what if we try to consider all of these things? What if we rank the most accomplished directors pound for pound, and then with an eye toward the context of their times, judging the levels of difficulty and sheer force of will to accomplish amazingly daring feats of directorial creation, as well as with attention to their impact on cinema history and other filmmakers, and also weigh the power of key works and greatness overall?
To me, a name that rises to the top when we combine different perspectives in this manner is Akira Kurosawa. His innovations, his eye for storytelling and for visual artistry, for symbolism and for realism, his understanding of the nature of film history and the progression of visual storytelling (particularly with regard to Asian cinema and the presentation of, for example, samurai tales and martial arts depictions), is a combination of talents far head of his time and nearly unparalleled. He inspired and influenced filmmakers around the world, in all manner of genres (inspiration from his films can be found in westerns, science fiction, horror, gangster, and war movies, to name but a few). He told stories that reflected the mood and important tides of social sentiment, sometimes giving voice to unspoken feelings and fears just below the surface of public feelings and behavior.
So Kurosawa will be my answer, while I tip my hat to Spielberg, Lang, and others who are all likewise pretty valid options and ones who come to mind depending on how I look at the question. I think that Spielberg, Lang, Kubrick, and Herzog all also have a very solid case as the right answers even in the overarching method of consideration I used to come up with Kurosawa as my answer. Asked on a different day, I'd probably name any one of them at any given time, too.