Design-wise, Emboar, Cryogonal, Klinklang, Thundurus, Tornadus, Landorus (in their Incarnate Formes) and Garbodor could make a grown man cry from their sheer ugliness.
As for stats, moves, etc. (basically Pokémon's worth in a battle), I don't like the elemental monkeys (sorry, I just can't see anyone using them after being done with the Striaton Gym), Archeops because of its terrible ability, and Stunfisk.
Zorua being event-exclusive was very disappointing. That was a huge letdown, especially after how much Game Freak tried to market Zorua and Zoroark... they were the first Gen V Pokémon to be revealed to the public, for crying out loud.
And while this doesn't affect my opinion of this Pokémon, I felt a bit cheated when I found out Terrakion isn't a Ground-type... it's a Rock/Fighting-type. That's a deceiving name you have there, Terrakion.