I am nominating Slurpuff for, at best, B- rank and at worst, C- rank.
Why? If you don't know, Swirlix got banned in LC because Belly Drum plus Unburden plus Berry Juice was OP as it gets. After that it doesn't even matter if you resist its Play Rough! Then the stupid thing gets Cotton Guard and Calm Mind and can maintain with its STAB Draining Kiss! Slurpuff can perform the exact same set in OU minus the instant full recovery granted by Berry Juice.
Slurpuff isn't as good as Swirlix because LC is a whole other game, but the set is still somewhat impressive for cotton candy.
Sorry, but this is OU, and nobody cares about Swirlix. Seriously, try running Belly Drum on Slurpuff and see how far you get. Besides that, now let's get to the actual countering of your argument.
"After that it doesn't even matter if you resist its Play Rough!"
Umm, I'm sorry, but that is
completely false.
+6 252+ Atk Slurpuff Play Rough vs. 252 HP / 0 Def Scizor: 232-273 (67.4 - 79.3%) -- guaranteed 2HKO
Remember, this isn't Mega Scizor, who has 40 more points in physical defense. Just regular Scizor, 0 defense investment. If Scizor tried to revenge kill Belly Drum Slurpuff, it would succeed. It could swords dance on the play rough, survive easily, and OHKO with Bullet Punch.
+2 252+ Atk Technician Scizor Bullet Punch vs. 4 HP / 0 Def Slurpuff: 488-576 (159.4 - 188.2%) -- guaranteed OHKO
I think you can now see how Scizor, a common OU threat, completely stops the Belly Drum set. Tell me if there is something I missed, and Slurpuff can win (I promise I won't be an arrogant anti-Slurpuff bigot :). I just don't see a way, though. Another common OU threat that stops this set cold is Heatran.
+6 252+ Atk Slurpuff Play Rough vs. 252 HP / 0 Def Heatran: 110-130 (28.4 - 33.6%) -- 97.2% chance to 4HKO after Leftovers recovery
Meanwhile,
252 SpA Heatran Flash Cannon vs. 4 HP / 0 SpD Slurpuff: 332-392 (108.4 - 128.1%) -- guaranteed OHKO
I understand Flash Cannon isn't a very common move. So,
252 SpA Heatran Lava Plume vs. 4 HP / 0 SpD Slurpuff: 166-196 (54.2 - 64%) -- guaranteed 2HKO
He can also destroy you with Will O Wisp, as well. Keep in mind that Lava Plume has a 30% chance to burn.
Heatran also stops the CM set, because he resists Draining Kiss and is immune to Flamethrower via Flash Fire.
+6 252+ SpA Slurpuff Moonblast vs. 252 HP / 0+ SpD Heatran: 110-130 (28.4 - 33.6%) -- 97.2% chance to 4HKO after Leftovers recovery
Draining Kiss does less. I don't even know if Slurpuff gets Moonblast, but Draining Kiss didn't even show up on the calculator.
Taking neutral damage from all hazards, slur puff can easily be phazed out by Pokemon like Skarmory, as well. I would put it at
C, on par with Smeargle.
EDIT: Denied an OU analysis? Surprising. Guess I can't rank it then.