I prefer Rayquaza over the other two, personally. It's got quite an intimidating design (which I really like), and it could fly without wings—just like one of those Chinese dragons. Isn't that awesome? Also, this Pokémon's better than Groudon and Kyogre statistically. Rayquaza has a variety of moves in its arsenal as well, giving you more freedom while building movesets for it. Its main weakness are ice-types, but it's still able to overcome them with attacks like Fire Blast and Flamethrower. Rayquaza is one of my all-time favorite legendaries; I've been mesmerized with it ever since I had first seen this Pokémon in Ruby. (: