Okay, disregarding the very VERY obvious of unown for a second...there's not a lot of Pokemon I really dislike. But, looking down the national 'dex, I guess apart from Unown, the answer would probably be...chatot.
I. HATE. CHATOT. I hate hate hate the fucking thing so much. The fact that you have to rely on one of them in this game to save you is just *vomit*, and the one that Taka uses makes me want to exterminate the entire species even more.