This is a really tough choice for me.
Scrafty is an unstoppable beast with "Bulk Up (or Dragon Dance)", "Drain Punch", and "Moxie", but there's one thing that makes Zoroark such a nasty force on par with Scrafty and that's "Illusion".
So as tough as this choice is... I'm going to say Zoroark. He's a great deceiver, easily confuses opponents, and his "Illusion" ability gives him a higher survival rate than Scrafty on turn one.
Also, Zoroark IMO is the perfect standard for Dark Pokemon. I look at Zoroark as the Master of Deceit, and I like to think that Dark Pokemon are mainly focused on Tricks and Sabotage. Zoroark also has a very comfortable and fairly diverse move pool, he plays like a pro on a few different competitive teams I've used.