As much as I love James Cameron, I have to go with Ridley Scott.
The man is an amazing writer/director, and has contributed more masterpieces to the motion picture mainstream.
Alien, Black Hawk Down, Blade Runner, and
Hannibal are each vastly different genres of film, all of which are immense achievements in film-making.
Also, some of which both inspired and paved the way for other film-makers (such as Christopher Nolan of
Batman Begins and
Memento fame.)
TO his credit, Cameron is also amazing, and did create
The Terminator and The Queen Alien, but other than that, his career has slowed to a snail's pace. Whilst Ridley Scott continues to shine, and do what he does best.