I have not seen every film unfortunately (especially the pre-1970 ones) however, I will offer my choices based on the following main criteria -
1 Consistently makes films that I enjoy and remember.
2 Makes films that other people very different to me also enjoy and remember. (It is impossible to please everyone)
3 Able to accomplish a great deal with very little. (Bang for Buck although I do not mean bang literally)
4 Able to make great films without great actors (This is where I have to exclude Scorsese as I believe De Niro is the greatest actor of my experience and I am not sure how much is attributable to him - I have yet to see Gangs of New York. Same thing goes for Coppola - De Niro, Pacino, Duvall, Brando and Sheen - and I do not particularly think much of his non-Godfather films)
5 The film made is brave or ambitious. (Fincher I cannot include because of Panic Room and Alien 3)
So here are my choices -
James Cameron - Terminator, Titanic ( not another Titanic remake - he must be crazy)
Ridley Scott - Alien, Bladerunner
Roman Polanski - Chinatown, Rosemary's Baby, The Pianist
William Wyler - Ben-Hur, Roman Holiday
Hitchcock - North by Northwest, Rear Window
Billy Wilder - Some like it Hot, Double Indemnity, The Apartment
Victor Fleming - Gone with the Wind, Wizard of Oz
Mel Gibson - Braveheart, The Passion of Christ (He has only done 3 films and even The Man Without a Face is a good)
Oops that's eight directors.
Honourable mentions go to -
Wolfgang Petersen - Das Boot, Troy ( It IS an adaptation after all )
Luc Besson - Fifth Element, Leon, Nikita
Spielberg - Raiders of the Lost Ark (Think about it)