Of course it's going to be Froakie, but only because everyone already knows what it turns into. That being Greninja, of course. I know people like to think they already liked it and like to pretend he was never promoted until he got popular, but for some reason Sakurai picked it over the rest before the game even released. There was some reason why Greninja got a clearly much superior ability over the rest. There was definitely a reason why Greninja got the starter role for Ash in the anime too and there was a reason why he got a bond form over the rest. Even its theme stands out compared to the rest. You've got a knight, a mage and of course, the ninja, which Japan adores.
Chespin's still my bro though, it definitely ain't some Twitter poll that's going to change that. I don't really mind people liking what they like, but at the same time, I can't really say there wasn't a bias over the line. But, each starter fell into a role and between the fire mage fox and the water ninja frog, of course the grass knight pangolin/armadillo/hedgehog felt the most unattractive when compared to them. The other two simply felt cooler to many compared to the bulkier, slower armor clad shrew. I'm coming out and saying this as a Chespin-Quilladin-Chesnaught fan, who isn't really afraid of what others might think of it.
