I think I'm going to go with Umbreon. I use one every chance I get, and I'll be transferring my girl one, Specter, to X when it comes out. Y will be the one I replay again and again, whereas X will be the one where I keep all the Pokemon I'll want to transfer to the sequel game. Umbreon is also easily my favorite Pokemon, I don't know how it happened... I guess between having Power Play be the first episode of the anime I watched as well as having an unintentional Umbreon come out of the Eevee I had in Emerald, my first Pokemon game, there's a nostalgia element to it. However, all nostalgia aside, I still like the fact that Umbreon can learn both of my favorite favorite attacks, has more of a defensive base stat, and has rings on it that resemble solar eclipses. I also like the fact that Umbreon, like all the Eeveelutions, is based on a feline/fox.