From the films I have watched (which are not many from before the 1950's), which I would rate as best of the decade. Pretty much, if the director made a film that stands tall above anything else that I have watched from the decade, it is the best director of the decade, which happened in the decades I have an opinion on the matter.
1920's -
Carl Dreyer
Because:
Passion of Joan D'Arc Best 1920's film I ever watched, by far. Though I haven't watched many.
1930's -
Fritz Lang
Because:
M Again, I haven't watched many but this is my favorite by a substantial margin from the bunch.
1940's -
Yasujirō Ozu
Because:
Late Spring. Easily my favorite movie made before 1950, Late Spring embodies Ozu's domesticity and is his most powerful film (from my POV). Truly a masterpiece and almost the ultimate drama film.
1950's -
Akira Kurosawa
Because:
Ikiru. My favorite film that was actually filmed in a continuous fashion and uses actors (instead of frame by frame and animation cells), Ikiru is the best conventional movie I ever watched by a substantial margin. Indeed, the greatest movie of the greatest director (from the point of view of his influence over modern cinema).
1960's -
Stanley Kubrick
Because:
2001: A Space Odyssey and
Dr. Strangelove, also, Lolita is excellent as well, though not absolute classic material.
2001 is without a doubt the best Hollywood movie of all time and Kubrick only English or American director on my list here.
2001 is easily the best of the decade and also Kubrick's best by a comfortable margin.
1970's -
Andrei Tarkovsky
Because:
Stalker and
The Mirror. These two movies are 2 of the 3 favorite movies from the 1970's, easily the two best art movies ever made, in my book.
&
1980's -
Hayao Miyazaki
Because:
and
1990's -
Hayao Miyazaki (again
)
Because:
and
2000's -
Hayao Miyazaki (and again
)
Because:
2010's -
Hayao Miyazaki (expected
)
Because: Given that he reached 69 years old in 2010 and life expectancy for a Japanese of 69 years old is currently around 90 year or more and given that he continues to work producing films of the quality he did in previous decades, Miyazaki will continue to be, without a doubt from the point of view from my tastes, the best living director.
I already expect The Wind is Rising to be the best movie of the decade, given that it is Miyazaki's most ambitious project since Mononoke and given the total incapacity of other artists (ANY others) to ever approach his work in terms of emotional richness (from the point of view of my current tastes in audiovisual media, of course
), he is without competitors, so as long as he works he will be the best director of each decade that comes.