Kanto: Squirtle, It looks so adorable and I love Wartortle's design, and Blastoise >>>> Charizard, sorry.
Johto: Cyndaquil, same as Squirtle, I love middle starter evos, they look really cool tbh, and Quilava is one of them.
Hoenn: Treecko, but there's a deep reason for that. I actually started playing Pokémon with Platinum and I played PMD EoS before any other game of the main series. Grovyle is one of my fav characters in the game, and that's why I choose Treecko.
Sinnoh: My first starter, who will be in my heart forever, Piplup. I actually don't like Prinplup so much, but Empoleon is DA FREAKING KING.
I like Turtwig and Chimchar so much, If Piplup wasn't my first starter, this choice would be sooo difficult.
Unova: Oshawott, same reason as Cyndaquil and Squirtle.
Kalos: When XY was announced, I would choose Chespin :3, then Quilladin was announced and made me love Chespin more, but then Chesnaught was revealed... broke my heart. So I'd go with Froakie