Mine was actually Zapdos as well. I'd grown tired of having Pikachu on my team in Yellow, and what better Electric Pokemon to replace it with than the most powerful of them all? Well, at that time, at least.
I also liked Mewtwo, but with the way that Psychic types were overpowered in the first gen, I didn't even really want to use it, since it could wipe through the Elite Four basically by itself.