Joel and Ethan Coen - NO COUNTRY FOR OLD MEN
Tony Gilroy - MICHAEL CLAYTON
Jason Reitman - JUNO
Paul Thomas Anderson - THERE WILL BE BLOOD
Julian Schnabel - THE DIVING BELL AND THE BUTTERFLY
This was a tough one, but I give the nod to Paul Thomas Anderson's There Will Be Blood. The Cohen Brothers were excellent, but so much of the action takes place off camera in the film. Anderson's film is an epic that follows the debased oil tycoon through 30 years of his life, showing how he waged his war first on man, then God, until he was "finished". This is a classic film by one of the best young American directors today.
UPDATE: The Coen Brothers won. I thought that Paul Thomas Anderson would win this one so that No Country and Blood would split the vote but I was wrong. They did an amazing job though.
You are what you like.