Definitely Alfred Hitchcock. He understood better than anybody how to manipulate suspense and keep the viewer on the edge of his/her seat. Take the scene in North by Northwest when Cary Grant is in the middle of nowhere. Hitch has him standing, waiting for maybe five minutes and you know something's going to happen and still he makes you wait, pushing it to the edge, building the suspense until something finally happens.
Guillermo del Toro is brilliant....Pan's Labyrinth was amazing.
Favorite director....that is hard...Sam Raimi (Evil Dead series...not so much Spider-man), Peter Jackson (Lord of the Rings was spectacular in regards to cinematography), Timur Bekmambetov (Night Watch and Day Watch) and Coppola would have to be favorites...
TIM BURTON all the way!!! I was always enchanted by his films. The nightmare before christmas is a beautiful classic. I cannot wait to see 9!!! (release date 9-9-09) how catchy! I'm literally jumping out of my skin waiting for his version of alice in wonderland.
Martin Scorsese. If you watch "Raging Bull", it is literally poetry in motion. Not to mention, he has made some of the greatest films of all time. His directing style is extremely polished and always new and beautiful to watch.
Quentin Tarantino, P.T. Anderson, and Sergio Leone are honorable mentions as well.
P.T. Anderson is probably my favorite contemporary director. Guillermo del Toro is really good as well, but only has 2 movies that I believe really excel (Devil's Backbone and Pan's Labyrinth). Peter Jackson has a fantastic directorial eye, but I fear he tends to be a tad bit too sensationalistic and superficial for my taste.
Best of all time? Probably Stanley Kubrick. His best movies have a very signature style but are nothing alike (Dr. Strangelove, 2001: A Space Odyssey, The Shining, A Clockwork Orange, Full Metal Jacket). He's truly legendary.