I cant really pick out just one.. and its mostly a close call between Zoroark, Bisharp and Golurk.. mostly for different reasons..
Zoroark is such a fantastic dude, with his ability.. you can easily mask him almost prefectly as a different pokemon on your team.. with like corresponding moves and all of that.. and your opponent would be none the wiser. I dig stuff like that.
Bisharp has an interesting typing that works well for me in most situations. In Black, he has basically my signature false swipe/catching guy. He also looks pretty amazing in X/Y with blades that retract in and out of his arms.
Golurk just looks cool.. and also happens to have a pretty good typing.. at least when it comes to offense. Its just a shame that while he has a cool ability, it's always unstabbed and his stats are a bit of a disappointment. My Golurk in X/Y made it all the way to 100, which would be the first time in years that I actually leveled someone to max.. but that was mostly because of horde encounters and earthquake.. and also having the AI continue to try to use moves on him that don't work (fighting/normal and the like)