It really depends on what the rest of your team is, as both Greninja have different uses. I’ll tell you what worked from personal experience.
HA Greninja to me felt like despite its types still lacked defense a little. So I tried Ash Greninja.
Ash Greninja is obviously better than protein in stats. It has higher speed and really close special attack to Mewtwo, one of the greatest standard sweepers of all time. Ash a Greninja has access to STAB priority moves such as water shuriken and STAB reliable moves such as Dark Pulse, perfect for a sweeper. Also, if you are wondering how hard it is to activate battle bond, I wrote a short experience about my strategy.
My pro strategy is to abuse Ash-Greninja’s insane speed, it’s usually able to out -speed most Dragon and Ghost types (I choose those because they are really common). I have it hold Iceium or Darknium Z depending on what’s more popular (Dragon or Ghost or Psychic type). Make sure to get that Greninja in a place where it’s Z-Move is super effective (Sounds really hard and situational but it’s not really, due to the amount of ground/dragon type users). You use the Z-Move and it should OHKO. Greninja battle bond activates and you now have a super powerful Pokémon and an OHKO on your opponent.
The strategy worked well for me.
Final note: some may argue Greninja is supposed to be unpredictable, and nobody will suspect Z-Move Greninja.