I've really been enjoying using Ash Greninja lately. Its able to acquire so much free damage early / mid-game thanks to the threat of Battle Bond forcing out low health / death fodder Pokemon, allowing it to weaken many of its checks with respectably powerful Dark Pulses and Hydro Pumps. Once Greninja transforms, its the ultimate cleaner since it outspeeds a majority of the unboosted metagame and 1-2 shots everything with its frighteningly powerful Hydro Pumps and Dark Pulses. Greninja also has a very powerful priority Water Shuriken against faster mons like CS Genesect and Pheromosa, meaning they are hardly safe responses.
Greninja has some issues with Fat Grass and Water types like Mantine and Tapu Bulu, but pairing it up with things like Tapu Koko, Celesteela, and CB Genesect can usually resolve these issues.
I'm glad to see other trainers consider how Ash-Greninja could be viable competitively. I've been using him as a poor-man's Pheromosa in game because everybody's talking about him being a glass cannon, but I'd like to really look into what he can do. Can we get a thread devoted to him specifically?