Too many, but here's a few...
Fritz Lang - Metropolis, M, Dr Mabuse, oh and he also invented film noir... the most important director ever IMHO
Alfred Hitchcock - he was every bit as good as his reputation
Otto Preminger - another great hardboiled director, but far more than that
John Huston
Akira Kurosawa - even his later films are mesmerising
Lindsay Anderson - the best British film director ever?
Alan Pakula - particularly The Parallax View...
Werner Herzog - the craziest, most egotistical, most brutal... but what films!
Francis Ford Coppola - the best US director of big movies of recent years
Bertrand Tavernier - L.627 is the best cop film of all time. See it if you don't believe me.
Jim Jarmusch - Ghost Dog is my favourite
Patrice Leconte - he is very sentimental and frankly wierd about women, but I love his work...
Spike Lee - such an amazingly talented director sometimes overwhelmed by his own politics, but anyone who can make films as diversely brilliant as Do the Right Thing, Inside Man and When the Levees Broke is alright by me...
Hayao Miyazaki - the king of Japanese animation
Coen Brothers - most of the time they are the bleakest but funniest author-directors around...
Ki-duk Kim - my current favourite Korean director
Kore-Eda Hirokazu - the best of a very good crop of young Japanese directors around. Try Afterlife, Nobody Knows, or Still Walking.