Keldeo. I lost to it twice at Winter Regionals because they lucksacked T2/3 Candy Blastoise 7 energy on a Keldeo. Also, at States, I started with Keldeo-EX (was playing Darkrai/Lasers) against Garbodor, that ended up costing me the game. In the next game, I was put to sleep with a Hypnotoxic Laser, I had the T2 Hypnotoxic+Virbank+Dark Claw Night Spear in hand as long as I could Ultra Ball for Keldeo-EX. It was prized. Needless to say, Keldeo doesn't like me and I don't like Keldeo.
What is your favorite concept for a Pokemon design in the video/card games?