Aromatisse: Immune to Taunt, has tons of support options, amazing bulk. Although Sylveon and Florges do bulk better and Klefki does support better, Aromatisse does both. This thing is definitely not NU, and probably has uses in higher tiers too. Too bad it's literally slower than Slowbro.
Klefki: Speaking of Klefki, here is a support mon, and a Swagger+Foul Play user that's better than Liepard (although it does Support better). The only thing it can't do in terms of support is Taunt...which makes it Taunt bait itself. But considering that its mission is to set up a screen or put up Spikes, a non-Prankster Taunter doesn't really ruin it that badly. Also it has the best defensive typing in the game, which is nice.
Mawile: Switches in with Intimidate so it has bulk. Mega Evolves, and then practically has Base 260 Attack at that point, the highest unboosted non-Choiced Attack stat in the game. It also gets Sucker Punch and Play Rough, which are both going to hurt. The only thing is that it takes up your Mega slot, but thanks to its power and it having the best defensive typing in the game, it will be worth it for a lot of teams. If Mawile doesn't make OU, its Mega Stone is probably getting banned in all lower tiers.
Sylveon: Amazing special wall, and can support its team with Wish too. It's no slouch offensively either, and it gets Moonblast and a ton of coverage options. Physically it can still tank some Fighting and Dark hits pretty well.
Florges: This thing is Sylveon with higher special bulk, lower physical bulk, Aromatherapy, less coverage moves, and a lower HP for Wish. Use either one as you see fit.
Togekiss: Jack of all trades, master of some. Which ones? Special walling, paraflinching, Nasty Plot, and Baton Pass, to name a few of its uses. One of the best Fairies in this Gen.
Gardevoir: Physically this thing has approximate bulk of a Blissey, which is usable...for taking resisted hits. It can take any Fighting-type move, as well as any physical Psychic-type move, but any strong physical attack not of the types Dragon, Fighting, or Psychic will make it keel over and die quickly. Mega Gardevoir is interesting, but Pixilate has little use, and there's a lot of other Megas you could be using.
Azumarill: Best non-Mega offensive Fairy. Choice Band hurts, and if you somehow let this thing set up a Belly Drum and you don't have something like Quagsire or Ferrothorn to revenge it, then you're dead. Even Ferrothorn is OHKOed after SR and 2 layers of Spikes.
Then there's other stuff.