Chandelure and Hydreigon.
I've honestly tried to like them both but I simply can't. In the case of Chandelure I already have a good, solid Ghost type on my team--my Golurk--and I can't justify putting a second Ghost on my team. I also have other Fire types that I rely upon, like Infernape so it there's no real point in using it. Hydreigon suffers from that as well; I have a Dark type on my team already and I don't use Dragons. I'm sure that they're both great, but I just can't get behind them.