As much as I love the Mingus band and Dave Holland's one time (for now) big project, and with all the respect I have for many of the others mentioned I voted to Maria Schneider.
That brings an interesting thought: two of the best band leaders and arrangers today are women (Maria Schneider and Carla Bley). Melba Liston also comes to mind when you talk about arrangers.
Why are women such good big band leaders?
Does this have anything to do with attributes like patience, frankness and motherhood?