This is my first post. I have been lurking, please alert me if I need to lurk moar.
Oh Greninja, I am very on the fence about this mon here.
I'll start out with his pros. Greninja is an absolute monster on paper (in practice too but we'll get to that). He has the coverage options to hit nearly every 2HKO every mon in the game. Directly switching into this thing is a scary thing, especially for heavy offense teams. His list of counters depends on his moveset, but determining what set it's running is never particularly easy. Which leads me to the ways of dealing with this thing.
1. Scouting- Mons with protect, double switches, and certain scarfers can help to reveal greninja's movepool and help you figure out what mons on your team can serve as a reliable check. For example, Sp.Def jirachi with protect can scout for dark pulse, if its there your venusaur may be a good check because few greninja pack extrasensory and dpulse.
2. Scarfers, Priority, and faster mons- I rank this strategy below scouting, because with scouting its movepool, you don't necessarily need to lose a member of your team. However, especially for HO teams, a sac n' switch to one of these mons is one of the only ways to handle greninja. That doesn't mean that its a bad one though. Many people I find, overestimate greninja's ability to just switch out when these threats are sent in. Sure greninja can just switch, but that means your opponent now has to deal with a scarf terrak, keldeo, etc, which is not exactly easy. Mons with priority and u-turn can allow you to build momentum against a greninja team. So even though gren can just switch, the fact that the opponent is having their mons weakened is being highly underestimated.
3. (By far the worst IMO) Wear it down- Greninja's life orb is, for some variants of stall, the only way to keep greninja from rampaging. With hazards and life orb and status, wearing out greninja is, in desperate times, a viable strategy.
Some arguments I've heard compare greninja to Mega Heracross, in the way Heracross devastates stall, Greninja devastates offense. While these arguments aren't perfect, especially considering greninja can often put in good work against stall as well, having mons that almost always ruin a play style seems to be totally ok when its stall, but when a mon comes through that ruins offense, people tend to be somewhat hypocritical.
Another thing to consider (and many do not care at all about this, but I'm certain some do), is the way the community looks to those outside of it. Greninja's popularity and relatively low stats, will undoubtedly create some uproar from casuals, and the banning of a mon that I find to be manageable, makes our community look a little ban happy, and unwilling to adapt.
That being said, are the adaptations that we must make over centralizing. From my personal experience I'd say no. My current team, carries none of the "standard" greninja checks, tenta, empoleon, chansey, but by scouting and strong, faster mons (that I probably would've used anyway), I've found myself rarely being swept by greninja. I've certainly lost a mon to it, but when I bring in the faster mons I carry, I find that greninja's ability to just switch out can often mean an opponent being placed in a position where I can sweep.
Overall Greninja is a monster, this cannot be denied, but I find playing around it to be easier than what many in this forum have been saying so far. So for me its a *no ban* but if it does end up being banned I could understand why.