If you can get Protean Greninja with a decent nature, that's considerably better, especially if it's transferred over from ORAS and has Gunk Shot.
Otherwise, stick with Ash-Greninja. Even with 10% less speed, it's still fast and hits insanely hard, especially since Water Shuriken is both priority and impressively strong (only STAB Extremespeed and Sucker Punch are stronger).