Clefable has 2 stellar abilities in Magic Guard and Unaware respectively, as well as a decent movepool that lets it perform a variety of roles, including but not limited to: Calm Mind Sweeping, Cleric, Tank, Anti Setup (Unaware), Wish passing and so on. With Magic Guard, it can soak Will-o-wisps and Toxics all day long, with access to reliable recovery in form of Wish as well as Moonlight.
Although we're yet to see just how good this thing truly is, after Pokebank released Pokemon take back the OU, it is safe to say that it's unique. Sort of. With Aurora Veil in sync with Snow Warning, this thing makes for a great lead for for Hail Teams, boasting STAB Moonblast and Blizzard (perfect accuracy). Thus it can take some hits with Aurora Veil and return fire immediately with decent Special Attack and Speed.
This thing. This thing right here. Wow. With Huge Power and access to Belly Drum, STAB Playrough, STAB Waterfall and best of all STAB Aqua Jet, it can ruin teams not prepared for it in a flash. Add in a Choice Band and Superpower and you've got yourself one super hard hitting train of a Pokemon.
Although it's not officially released yet, Gardevoir and its Mega have a lot to offer. Standard sets include STAB Moonblast, STAB Psychic/ Psyshock, Thunderbolt and Hyper Voice (Works amazing with Pixilate as Mega) for good measure. With Synchronize as Gardevoir, it punishes certain status spreaders.
This unreleased Uber blew the meta game away with its Power Herb + Geomancy combination, as well as a great ability that boosted its primary STAB Moonblast to whole new levels of destruction. So much so that the only reliable way to counter it after Geomancy was to have Aegislash, Mega Gengar or Mega Rayquaza. Mostly the first two, though. Some people also used Genesect, but it's safe to say that to date it remains the hardest hitteg Fairy Pokemon. It can also run the role of a mixed sweeper and cleric as well!
With an amazing ability in Psychic Surge (Thanks, J2 :D ) and extra ordinary special attack (135), and above average speed, it can wreck entire teams all on its own. With Psyshock and Psychic getting a free boost, it is safe to spam Moonblast and Psychic/Psyshock till it ruins whatever hopes and dreams your opponent has. Further, the ability stops the use of Bullet Punch, which in itself makes it a whole lot more stronger. The Specs and Scarf versions of this has the capability to 6-0 teams without a counter against it.
Okay, so it's not as good as Tapu Lele, but with the right conditions, this thing is an uncontrolled beast! With a decent ability in Grassy Surge, it can decimate unprepared teams with its monstrous Attack. It takes some practice, and is now countered better, but every now and then, it shines through by taking hits like a boss and making the opponent suffer the wrath of the bull.
With a super strong ability in Soul Heart, this slow, cute looking Pokemon can dismantle teams without mercy. It has a really good typing and the right moves to knock your opponent down, i.e. Flash and Fleur Cannons.
- Klefki was great, and will probably still be, but with nerfs to Prankster, T-Wave and Toxic's accuracy, it is slightly lesser of a threat now than before.
- Mimikyu is a decent Pokemon with a gimmicky kind of ability, but with at least one Swords Dance and access to Shadow Sneak, it can hurt opposing Pokemon. Unfortunately, it has low HP and its typing could be better, so it loses out when battling in the higher leagues of competitive play.
- Florges has great Special Defense and access to Wish, but that's about it. It is easily countered by Physical Steel mons, so it's not much of a threat
- Sylveon is really strong, but it suffers from the same troubles as above.
- Tapus Koko and Fini are both great, in OU, but compared to the other two, they don't stand out as much. Specially Koko, without strong STAB moves. Of course, Roost is great, but that's about it. Fini on the other hand is excellent as a Special Tank, but I've not seen its use like I have the other two, I may be wrong here (haave been before about Tapu Lele), so feel free to correct me.
Whew, that took a while. Hope that helps out for now. This is probably the longest answer I typed (all original roo :D) so I hope it's as helpful as I want it to be. Cheers!