Perhaps not the _greatest_ directors ever, but the ones that mean the most to me:

1. Louis Malle
2. Lindsay Anderson
3. Francois Truffaut
4. Andrei Tarkovsky
5. Ingmar Bergman
6. Werner Herzog
7. Krzysztof Kieslowski
8. Stanley Kubrick
9. Roman Polanski
10. Bill Forsyth

Others which coulda been contenders:
Rainer Werner Fassbinder
Jean Vigo
Nic Roeg
and more.