Design-Wise: The whole Fennekin Line.
Are you trying to mock my favorite Pokemon? Never has my Delphox fainted once, even though it got some nasty hits, like Zygarde's Earthquake, and it's a complete wall for catching Mewtwo. It's a wizard's robe for the reason that it pretty much is one, as a Psychic type. Though I do agree that it could be designed slightly better, I still love it. Augmenting fiery abilities with the mind... that's a deadly combination.
By the time I saw a Diggersby, my Braixen laughed at it after delivering a Psyshock. Then there's Shalour Gym, which was simple for Braixen. It may have been target practice for a slighly inaccurate move like Psyshock (literally, controlling that much at once with with magic isn't easy).
The legendary Pokemon are as unique as I've seen them in other games. Even though they're totally different, they're still unique. The only problem I have with Xerneas is finding its unique move. I run Horn Leech, Moonblast, Megahorn, and Night Slash. The downside I saw with this was that Xerneas is actually Special Attack based, not physical. But it's hard to NOT resist 356 Attack. Take damage? Simple solution: get an opposing Pokemon weak to it, do as much damage, then heal that damage you take.
Klefki, used by Wikstrom, is a problem for myself due to Spikes getting priority. All I can do by the time I battle him is that he doesn't deploy them. I need Haxorus' Mold Breaker Earthquake for ignoring Sturdy (because it was of little use back then, they decide that they can't let it be one-shotted without special methods).
I do believe that Barbaracle could use some improvement. It's, for me, ridiculous in its look, and it's way too easy for someone to beat as an Elite Four Pokemon. I give Delphox Grass Knot for obvious reasons for covering weaknesses, but it is just something else IF I could see it attack.
Against most people, as I've read so far, I actually don't mind the Flabebe line much. It completely caught me off-guard that it was a Fairy type, and not Grass. I'll compliment that as a tricky way to interpret Pokemon, especially when you first see them.
And Talonflame is a Pokemon I like, but with extreme risks, as I've seen against Malva's own. For Malva's Talonflame, it seems the only move it knows is Brave Bird, wrecking your team while killing itself, trying to do as much damage as possible before it goes down, which isn't hard to do so.