Cyndaquil from HeartGold. I don't want to transfer her because she has all the pokeathlon medals xD She actually was my strongest when I faced the elite four ^u^
Torchic from Emerald Version! Then Turtwig in Platinum, Chikorita and Squirtle in Heartgold, Tepig in Black and Black 2, Froakie in X and relived nostalgia by picking Torchic in Alpha Sapphire!
To be fair, I've gone back and played others since! I wish I played them back when I was a lot younger and had a DS lite for the first time, but to be fair I never got it as a gift (instead I got horrible "girls" games, I wish I had Pokemon, would've loved it) and to be fair I liked the anime just fine and thought a lot of Pokemon were cute but somehow never knew what the games were like and I just assumed I wouldn't enjoy them.
I guess it's okay that I don't have childhood memories of the game, because I certainly do with the anime/cards thanks to family/exposure. I was born in 2000 so what can you do tbh.
My first ever one was Bulbasaur in Fire Red! I remember picking him because Professor Oak said it'd be the easiest to raise or something haha and I was a little kid.
my very very first was a cyndaquil on gold (it was battery-less and i never figured out how to get past goldenrod until after i replaced the battery), but the one i consider to be my first real starter was a bulbasaur named "Venus Trap"
i wasn't able to beat the elite four until it was level 93 and my frenzy plant was able to kill agatha's gengar and lance's dragonite ahaha