Mew, Giratina or Sky Shaymin... Giratini because it's the first legend I caught in my first Pokemon Game! Mew, because back then, the internet didn't exist... and Mew was the biggest MEWstery! Shaymin is so cute!
Lugia! after seeing the Pokemon 2000 movie, Lugia was definitely my favorite, and has kept the position since then. he is beautifully designed with elegant colors and I absolutely love his back story. Suicune is a close second, though.