As for the topic at hand, I'm going to go in a different direction and tackle a few current guys. They may not be giants in their field yet, but they should be and years from now I hope their work will be looked at as classic.
Alexandre Aja, a name that most people don't know but they should. While hacks like Eli Roth are being lauded for pure crap like Hostel and Hostel 2 Aja has taken the horror genre and returned it to what it should be, horror. He sidestepped the gore and gore alone "new horror" and made Haute Tension and The Hills Have Eyes, two of the greatest horror movies ever made. They are full of gore, but the gore is only there to add to what makes horror movies great, the suspense and the mystery. Aja understands this and that is why his will be a great director for years to come.
Guillermo Del Toro, the man does one thing and one thing only, and that is make masterpieces. His greatest work is probably a movie that almost no one has seen, and that is The Devil's Backbone. Then he followed that up with the criminally underrated Blade 2, the beyond excellent Pan's Labyrinth and the best comic movie to come out in forever, Hellboy. With Hellboy 2, The Hobbit, and other fantasy work such as At The Mountain Of Madness and Dr. Strange on the way this director that gets that fantasy is adult like no other won't be slowing down anytime soon.
Matthew Stover, most people know him for his Star Wars work, but those, while great, are tamed down versions of what he is capable of. The Caine Trilogy offers a grim and dirty look at the fantasy realm where there aren't any whimsical fairies or happy endings, rather people die, end up crippled, and hate their lives. His SW novels have been so dark and yet so full of character and intense plot that he leaves any other SW authors not named Troy Denning in the dust and makes them look like the average authors they are. His work in fantasy should become the new standard for adult fantasy in prose, it won';t, but it should.