Rayquaza. Do I even need to say why? HE IS AWESOME! The shiny palette then? OMFG, the best I've seen. He started the Dragon-type legendary Pokémon series which hasn't ended since then. He has the main role in the games, stopping Kyogre and Groudon, and well, he's just my favorite legendary (except for Lugia, whom I'm not counting because I'm only considering non-event legendaries).