Greninja. It's in Smash, it's in the anime, it's just everywhere. It was so well-received upon reveal as well, especially compared to Delphox and Chesnaught. Plus it was voted the #1 favorite recently, I believe. So yeah, definitely the most standout Pokemon for Gen VI, imo.
Froakie, Fennekin, and Chespin are also big, as starters always are. But it's been a long time since a final stage starter was literally everywhere.