It's a tough category but I'm giving this one to Mark Boal for Zero Dark Thirty. Other top contenders include Quentin Tarantino (Django Unchained) and Michael Haneke (Amour) but my money is on Boal.
Boal owon the WGA in the same category and the former journalists screenplay has been heavily praised. The nominees in the category are:
John Gatins, Flight
Mark Boal, Zero Dark Thirty
Quentin Tarantino, Django Unchained
Michael Haneke, Amour
Wes Anderson and Roman Coppola, Moonrise Kingdom