Back in the days of Pokemon Yellow, my favorite was Zapdos, simply because it was an upgrade over Pikachu, and nothing more. For the sake of the poll, and the fact this is about the original games, I voted for Zapdos.
Currently though, my favorite is Articuno. It's got the best design out of the three in my opinion, and the best typing of Ice/Flying. Not only that, but I love its moveset as well (whoever came up with the evil concotion of Mind Reader and Sheer Cold is some sort of evil genius).