The Imposter
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I think Greninja's problem it's predictable. Goodra owns it because the obvious ice beam is checked by a flamethrower.
The thing is that Greninja is good at what it does - forcing switches and gaining momentum on them. Not only is it a decent wallbreaker, it also serves as a scout; I personally use it as a way of scaring out the counters to my main sweeper and bringing in things it can handle.
The bans of Mega Luke and Genesect really helped Greninja, as they took the roles of "fast, frail sweeper that can gain momentum" and pulled them off better than Greninja ever could. It'll never be as good as people thought it would be when the hype train went through Kalos, but things are looking up for it.