Bulbasaur. Years ago, I mentioned he looked like a garlic bulb so my kid gave me her plastic toy Bulbasaur and now he sits on the windowsill as the patron saint of my kitchen.
As a child I trained a Kadabra to level 100 because another kid told me this is when it evolved.
Nope! We did eventually trade back and forth - but those 2 minutes of my newly evolved level 100 Alakazam on my friend's cartridge demanded an insane level of trust between two 10 year olds. That shit was intense!
I honestly can't choose. I love too many of them. Some of my favorites are clefable, pikachu, vaporeon, palafin, armarouge, absol, meowscarada/sprigatito, grovyle, corviknight, ampharos, wooloo... just too many!
It's not strong at all since it's a pretty early route mon, but definitely the buizel line. Grafaiai just barely first place for me, but I absolutely love that as well.
Umbreon! I have a tattoo on my forearm, but I got it as a poor and dumb college kid, so I'm getting it reworked in September by an artist that will do it justice!
Totodile. Elementary school me picked him at random when I played HeartGold (still hadn't wrapped my head around the whole touchscreen thing IIRC) and I loved the little guy.
My favorite Pokémon has been poliwag ever since I played Pokémon blue for the first time. He so adorable and becomes a very powerful Pokémon. I remember catching him early on in Crystal version and making him my starter Pokémon.