What is the most underrated and overlooked Pokemon that YOU like?

Ditto

When Greninja got a new anime form directly connected to the most well-known human in Pokémon and heavily featured in the anime, its new form was free in the next game (SM) and more powerful than a Mega (and didn't require an item and gave it stats above a pseudo-legendary's), and when it had to be removed due to Ash leaving, it got a Mega in the soonest game possible, almost like compensation, and was obtainable at launch. The others initially got nothing until they were forced to add them because it's Kalos again and they can't only give one of them a Mega. Plus, people would've continued to complain about them not getting Megas while being the starters of the gen that introduced them.

When you search Greninja on the Pokémon Center website, you get 31 results, but for the other two there are only 3, one of them was made because they had to include them and had no other option (Sitting Cuties), one of them is more like merch for the unevolved form, and the other is just a normal plushie.

Greninja has a whole 3DS theme/background for itself alone, something very few Pokémon have.

They treat the Kalos starter trio like it wasn't a trio but just Greninja.

When Delphox got a Mega, it felt more like obligation given the circumstances, like they couldn't not do it because Megas have to be for all starters, unlike alternate forms which can be given to one only like it isn't a starter trio. We had to wait an extra month for its online-exclusive Mega when Greninja got its on release, and Chesnaught had to wait two. At least Delphox and Chesnaught have better designs.

In the end, Delphox is the most perfect Pokémon ever made and horribly underrated and underused. Delphox and Chesnaught in comparison to Greninja get treated worse than Venusaur and Blastoise in comparison to Charizard.

That's a really good choice!

I like the evolution to it (I don't remember the name, though…)