Let's talk about Greninja
It got a boon with the addition of Gunk Shot from the move tutors. It has users resort to very sub-par checks such as Tentacruel and Empoleon, just to try and counter it. The fact that a lot of moves he has are very usable with Protean giving extra STAB, it's very dangerous. Essentially Greninja can fit into almost any team thanks to Protean not restricting it's typing.
People could argue that Priority kills frog and...It's true it does. Breloom and Talonflame are very popular among showdown users and are very reliable for clearing out Greninja. There is also a major 4 itis problem. With the set that Skitty gave above, you still run the risk of getting stopped by M-Venusaur, M-Ampharos, Chansey, Porygon2, Defensive Charizard-X, Gyarados, and Rotom-W. Worst, Rotom-W is extremely popular and has momentum with Volt Switch or can punish switch-ins with Will-O-Wisp. The meta has already been shifted with Greninja and Gunk Shot only detracts a few things now. Does it make it more unpredictable? Yes. Does it make it an automatic win-condition for every match? Hell no.
There is a bit more into Greninja such as the use of Water Shuriken and other unique tricks he has but, tbh, that has never really been a problem here. Most people are running the linear Special Greninja or occasionally the U-Turn set. That's two sets folks with a variable that may or may not matter at all. You might compare this to Genesect, but here is another argument towards that.
See, Genesect was a problem because Genesect could pull off virtually anything without a problem thanks to it's amazing stats and Download. It also wasn't worn down by Priority, nor was it easily stopped. The only thing that could downright defeat Genesect 1-1 was Heatran, and then you could just U-Turn out into Landorus-T or something. Greninja does not have this luxury as his physical moveset pales in comparison to his special moveset. Using a Physical Greninja would seem downright gimmicky as you don't really get much power into it. Even with STAB Low Kick, you STILL struggle to 3HKO Chansey w/Eviolite.
Not to say that Greninja can't do that either as, hello item. But that's only a very rare variable compared to a massive constant to a meta game. I'm personally going to be against banning Greninja.