Probably Mewtwo...as a kid on the playground, you always hear everyone talking about getting mewtwo once they had beaten the game and how awesome it was. that built up the anticipation until you managed to get through the main game and explore cerulean cave and catch him for yourself.
The best part is...he is as badass as he was hyped up to be. I was not disappointed
Bulbasaur - being my very first starter, and my only plant starter throughout the generations. I really liked its design and still do, some of the current plant pokemon, while great to use, look kooky (like Breloom!)
Charmander - Loved this line and used it in every playthrough. Tried to get it on as many future generations as I could so I could use it as a replacement starter.
Gastly - Always preferred ghost to psychic pokemon. Used a Gengar in EVERY generation.
Articuno - Hard to find a decent ice type pokemon, which I like. Used a Dewgong to clear the E4 first time round :/ never let go of my Articuno after I got it, plus it had Fly as well which is used alot. One my favourite legendaries, so definitely nostalgic.
I guess Caterpie. I don't like it like I used to, but I still like my memories of loving Caterpie and hold a special attachment to Caterpie because of that. I loved Ash's Caterpie and I just love bug Pokemon in general. I always used to write original-trainer fanfics and I always wanted to insert a character based on me and they'd always have a pet Caterpie.
"Earth is a silly place. Half the world has no clean water. And the other half has so much, they poo in it."