I have two.
One is Avalugg. Not because I actually hate it, but it's just very forgettable and bland in my eyes.
And also, Greninja. Dear god. An earlier poster already explained why he's the worst though. But basically he's overrated as heck and doesn't even have a good design. Delphox and Chesnaught got absolutely no love or attention because of him. He gets a cool hidden ability, big movepool, A NEW FORM STRONGER THAN MEGA EVOLUTION- and Delphox and Chesnaught don't? Excuse me, but that's overrated in my eyes. GameFreak is practically pampering Greninja and it's why no one gives Delphox or Chesnaught the love they deserve.