Gonna have to vote ban here. Its pretty insane how throwing fluffy on any bulky mon lets it invalidate most physical mons in the game. I think balance teams are the best abusers of this rather than stall, being able to throw a fluffy zapdos or mandibuzz on your team and volt switch/u-turn out and gain momentum on so much shit is amazing. I think we all know how crazy the defense is, but just some examples:
252 Atk Life Orb Tough Claws Hoopa-Unbound Knock Off (97.5 BP) vs. 252 HP / 252+ Def Fluffy Zapdos: 126-149 (32.8 - 38.8%) -- 6.5% chance to 3HKO after Leftovers recovery
252 Atk Aerilate Pinsir-Mega Return vs. 252 HP / 252+ Def Fluffy Mandibuzz: 79-94 (18.6 - 22.1%) -- possible 6HKO after Leftovers recovery
Some of the strongest attackers in the game do jack shit damage. banhammer for a fur coat clone is well justified.
Some other thoughts on mons I've been using:
Magearna is a god. RegenVest is amazing, excellent bulk with a nuke in fleur cannon on par with latios's draco meteor. Literally the best dual typing in the game allows it to switch into so many things, and then it can use a slow volt switch to gain momentum at literally no cost because it regens back the HP. This one set alone is probably worth S rank, I haven't even tried a shift gear boosting set which has some nice possibilities like tinted lens with fairium-z. The aegislash of gen 7, basically no downside to running this and it fits on HO, balance, hard stall, even niche styles like trick room and rain.
New surge abilities are awesome, in particular psychic surge. A 50% boost is strong by itself for psychic types, but invalidating priority moves is such amazing team support. Latios the mamoswine check lmao
Grassy surge ferrothorn along with leftovers acts like pseudo poison heal, and in combination with leech seed ferro just doesn't die. As a bonus palpatine himself would approve of power whip's 50% power boost. I still prefer flash fire but if you have some solid fire resists this is a hilariously fun set.
Poison heal boosting zygarde was already a massive threat, now it can run mono-attacking with only 2 pretty rare resistances, as most grass and bug types also have secondary steel or poison types. Can totally also run a sub toxic set like gliscor from like 3 years ago with a stab that bodies anyone who can avoid poison. Might have to be looked at later.
Nice new bulkier tentacruel alternative for stall, although no rapid spin. What do you guys run on this? I personally ran unaware but I'm sure there's better sets out there
In terms of broken shit, aerilate is still borked like last gen. Dragonite, landorus-t and noivern all still do exactly the same as they did before, they need a ban. The first two are kinda held in check by fluffy but I don't want a broken checking broken situation, hopefully banning fluffy will prompt these getting banned afterwards.
Also stakeout might have to be looked at later. Not only does it shit on stall when used on something like choice band tyranitar, OHKOing skarmory for example, it gives pokemon the ability to completely destroy your own soft counters. This is totally nuts, completely and utterly insane. Especially for fast pokemon with mixed coverage like thunderus, keldeo, or latios, you just can't switch into them at all. I guess we will see how the meta settles but I don't know how long these will last.
I don't know if it's just my luck, but I've seen maybe one or two Fluffy users, neither of which were particularly effective. I personally don't think it warrants a ban when stupid stuff like the -ate ability abusers exist. 
