I'm having a hard time figuring this one out. The top three standouts seem to be:
Cluster Arrow with Grenadier passive, Impale with Sharpshooter passive, and Rapid Fire with Ballistic passive, but my mathcraft skills might be insufficient for determining the answer.
My fear with Impale/Sharpshooter is that for longer fights, the Crit chance dropping itself off from the Sharpshooter triggering and having to start over will lead to lower overall DPS but I don't have the formula to prove this.
With Cluster arrow, the main problem seems to be that with 50 hatred cost per tick, and average boss length fight will have you running on empty constantly.
That leaves Rapic Fire/Ballistic, but that answer has way too much gut instinct for me - does anyone know the numbers?