Kanto: I actually can't honestly pick a favorite from these three, they all are good, and I love all three. But if I had to say which one I'd lean towards the most of the three is Bulbasaur. But I don't love one more than the other, to be honest.
Johto: Always Chikorita. Compared to the other two, it's softer and sweeter looking, so I've always loved Chikorita more than Cyndaquil and Totodile.
Hoenn: Treecko is my favorite Pokemon of all time, and always will be, so I don't think I need to explain this one at all.
Sinnoh: Chimchar, and it's purely because of the anime and Ash's Chimchar's character development, to be perfectly honest. Turtwig used to be my favorite, but I honestly think how the anime showed Chimchar's progression as a Pokemon just made me lean more towards it as a favorite.
Unova: Oshawott. It's a goofy little otter, what's not to love? Dewott is also one of my favorite Pokemon.
Kalos: Fennekin, but I really don't have much to say about it because I found the Kalos starters to be a sad disappointment. I think Kalos was the game I ended up abandoning my starter in...
Alola: easily Litten, I like cats, although Rowlet is still p cool, I'm just not as enamored with as everyone else is. While I find Incineroar being based off a wresler not that great, it doesn't deter Litten for me.
Galar: Scorbunny, easily, I love rabbits. I don't know why people don't like Cinderace that much, I personally loved it and used it a lot in battle.
That's all I have to say, though.