Ash-Greninja is strong.... but its weaker than Talonflame and Hawlucha when its in regular form.
You can not be serious
When it was a Frogadier, it struggled against Meowstic.
It could only bring a hit against Mega Zard when the form was activated, while Talonflame brought in a hit against Mega Lucario, while not even fully evolved. Greninja couldn't take out Avalugg with the form (Although it wasn't fully mastered, it still had acces to the form), while Talonflame, without the form defeated it.
Wow, thats extremely biased
Not only is that Mega Zard an E4 level opponent while Lucario was a mere Gym Leader level one, but Avalugg had its Ice Body exploit while tanking Greninja's attack and didn't have his healing has when Talonflame got the win.
Talonflame brought a hit against Moltres and tanked Zapdos attacks, while Greninja hasn't faced legendaries.
Talonflame lost to both those Legendaries, while Greninja fought the Champ on equal footing and has a rivalry going with Mega-Zard, who is E4 level.
If you look it at that and if the battle of Hawlucha counts (Which imo, it doesnt since we didn't saw the whole battle and thus we don't know how many hits Absol got before going down to Hawlucha), Hawlucha is stronger than Greninja, since it brought down a Mega without having acces to the Ash-Greninja form, while Regular Greninja couldn't.
Hawlucha got oneshotted by Clawitzer, who then got oneshotted by Greninja.
People really seem to ignore the massive amount of wins Greninja has been having, even without transforming its clearly the strongest member of the current team.
Then add how we just saw Alain's Charizard wrecking Remo's team without Mega evolving and put up a good fight against Mega Garchomp before activating its own Mega Evolution, so even in "base" his Charizard is ridiculously overpowered (willing to bet its stronger then Ash's)
In their last battle Greninja went toe to toe with Mega Charizard, overpowered his first thunderpunch and then tanked 2 more attacks before even activating its Ash-Greninja form. So in its base it went toe to toe with the overpowered Mega who has beaten an E4 mega and just plowed right through every League opponent with a perfect winning streak.
Then we also have it putting up a good fight against Diantha's Gardevoir before both transformed
So yeah, Greninja in its regular form is already top tier and a possible contender for the nr 1 spot, as Ash-Greninja it is without a doubt the strongest