Mine has to be Mew. Groudon and Rayquaza are also amazing and I love the new Yveltal. Arceus and the creation trio are also something special due to their history and abilities.
Hmm. My favorite is probably Jirachi. It's so adorable, but it packs a huge power. Also, honorable mentions for Dialga, Rayquaza, Celebi, Mew and Xerneas. Also, other legendaries are cool too. :3