I mean...
It did just win the most popular Pokémon this year, so, y'know...
But on the other hand, a Pokémon with a cool design was outright let down by:
1. The fact it was special (Battle Bond) in a way no other starter has been. This is just pure stupidity.
2. It got the best hidden ability of any starter, Protean, which by the way is game-breaking in terms of STAB and type advantages. Cinderace got the same hidden ability with a different name, so that sucks as well, as it is unbalanced and unfair (it is pure favouritism).
3. Ash ruined it by taking a cool Pokémon, and using his chosen one magic to make it an overpowered monster. It won against the champion's ace, Gardevoir, who had a type advantage if it used fairy moves. And then lost against Alain's Charizard in which it was rigged towards Greninja in the first place! And then the Greninja was the key to saving the world, of course!
4. It got two signatures, Water Shuriken and Mat Block, which again is unfair. Similarly, Cinderace also got two: Court Change and Pyroball.
5. It got the best BST and Movepool to let it abuse Protean.
Anything else I missed as to why it is unfair?
It sucks that my favourite starter and one of my top 6 got ruined. Because of the fact that Greninja got overpowered, it meant that it is seen as unfair, uncool or cliché to like the Pokémon or use it in your team. This is just wrong in my opinion. Same for Lucario and Zoroark and Noivern and so on- their insane coolness and over-popularity made them stop being *your* favourite Pokémon, because they are everyone's favourite! You wonder why they bother making Pokés like Grubbin or Mr Rime if they know no-one will like them and clearly go for the only cool Pokémon from that Gen.