Of course it has to be Articuno. Great Pokemon. Wrecked Lance, I remember the first time i looked through the binoculars before fuchsia city..? Not sure of the correct spelling.
& seen "" in pokemon yellow i instantly fell in love. I remember thinking.. "now who the hell is that Pokemon?!"
To this day, Articuno is still one of my favourite legendaries.
NOTE: IF YOU CAUGHT MEW FOR OTHER REASONS THAN COMPLETING THE POKEDEX AFTER FINISHING THE GAME, YOUR A CHEATER.
( I liked everyones posts agreeing on the majestic ice bird also )