Bwind made a tournament last year on horror directors. I think Romero won it by beating Argento. Here's the link...
http://www.horror.com/forum/showthread.php?t=21982
And about my top 5 :-
1 - Sir Alfred Hitchcock
2 - Dario Argento
3 - George Romero
4 - David Cronenberg
5 - Sam Raimi
Hon. mentions - John Carpenter, Tod Browning, Mario Bava, Frank Henenlotter, David Lynch.
Thats for horror. Overall I d say Akira Kurosawa. The man was a genius.