Volcanion, not only does it look ugly, but it has no backstory in the games whatsoever, can't learn Surf and Waterfall for some reason, and is just really disappointing as the first Fire/Water Pokemon.
That being said, I also dislike Hoopa, Marshadow and Melmetal for being ugly (Meltan is somewhat cute at least), and Phione for being weak and pointless, although I'm not sure if the latter is even a Mythical.