This is my first time posting on this site go easy on me...
I have a few Pokes I'd like to see ranked, they've each served me well:
Florges for C Rank
Florges is bulky on the Special side, and her base Special Attack is pretty high for a defensive mon. This along with her access to support moves such as Wish and Misty Terrain could make her seem awesome, but her flaws definitely outweigh her perks. First of all, her physical bulk is just awful, Scizor or Metagross can easily come in and revenge kill her with Bullet Punch while she can't do anything back. She also has low HP for a Wish Passer, so she gets way outclassed by Sylveon that can do just about everything she can do, just better. The only perceivable way Sylveon isn't better is because Florges gets Aromatherapy, while Sylveon would have to wait for a Pokebank Eevee to be available for Heal Bell. Florges' last thing, which is what kills her the most to me, is that her abilities are completely useless in single battles (not to mention situational in Doubles). Without being able to abuse her abilities, she doesn't have a lot going for her, so Sylveon would always be a better choice, unless you really need a Fairy cleric, which is the only reason I ever used it.
Hippowdon for A Rank
Hippowdon got an awesome bonus this gen that increases his walling capabilities to me. Trapping moves were buffed, so he can essentially trap a grounded physical attacker with Sand Tomb, and the damage from that combined with Sand Stream damage earns a KO after just a few turns. He also has a great Attack stat, meaning he can do good direct damage if needed, and he gets reliable recovery in Slack Off, making him pretty hard to take down. He does, however, have his counters, notably BellyJet Azumarill, but he hasn't fallen from his fame acquired from previous gens as he still functions nicely in OU.
Quagsire for B Rank
Quagsire is so often overlooked, but he's very impressive. He is a very effective counter to sweepers that try setting up with Swords or Dragon Dance, notably Talonflame. Once he's in on these attackers, they usually try to set up more, a turn which can be used to attempt a burn (Scald) or trap them in with Infestation, which ends up being their downfall. He gets reliable recovery from Recover, so he stays alive for a long time if played correctly, and he has a very good movepool, which includes Yawn, Encore, Stockpile (amazing with Unaware), and even Haze. His downfall would be his bad special bulk and 4x weakness to Grass, making him easily phazed out by Grass-types and Special Attackers, but he fills a very nice niche in OU regardless.
Klefki for A Rank
I was appalled to see my keys in B- territory... Klefki has such an amazing movepool to abuse Prankster with, which is why I love him. He gets Swagger + Foul Play, which is evil in itself, combined with Thunder Wave... He becomes a very annoying Pokemon to deal with in any Meta game. His really impressive niche, however, is his access to priority hazards. Prankster Spikes ensures that he can at the very least get one layer set up. He also got a cool signature move, like most Kalos Pokemon. Fairy Lock is just a trapping move like Mean Look, but he abuses it nicely because of his typing. If he traps in a wall, ie Blissey, they can't escape the Toxic and can easily be stalled out with the help of Heal Block. He also gets Dual Screens, allowing him to support Bulky Offense teams very well. He has a niche over other supporters, and thus he is very deserving of being A Rank.