This is the most excited I've been about OU in a long time!
(All calcs here assume that people don't just start Terastalizing into defensive typings countering the below or trying to get bootleg Adaptability with their Tera.)
I think people are sleeping on Quaquaval; that defensive typing is surprisingly good, having Rapid Spin and Roost is pretty nice, the 80-BP Aqua Step boosting Speed Flame Charge-style gives Quaquaval a niche over Urshifu-R, and the dancer duck can check the very dangerous Chien-Pao quite well as long as that bugger doesn't carry Psychic Fangs or Acrobatics:
Quaquaval @ Expert Belt
Ability: Moxie
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
Tera: Steel / Fairy
- Aqua Step
- Ice Spinner
- Close Combat
- Rapid Spin
Use Life Orb if you want, but some of these calcs get pretty razor-edge with it in terms of surviving hits and not fainting while checking. Also note that you need Jolly to outspeed Chien-Pao after an Aqua Step.
Chien-Pao calcs assuming worst-case sane scenario of the equivalent of a -1 Def debuff:
252 Atk Choice Band Chien-Pao Crunch vs. -1 0 HP / 0 Def Quaquaval: 168-198 (54 - 63.6%) -- guaranteed 2HKO
252 Atk Choice Band Chien-Pao Icicle Crash vs. -1 0 HP / 0 Def Quaquaval: 178-210 (57.2 - 67.5%) -- guaranteed 2HKO
252 Atk Choice Band Chien-Pao Sacred Sword (135 BP to simulate -1 Def aura) vs. 0 HP / 0 Def Quaquaval: 250-295 (80.3 - 94.8%) -- guaranteed 2HKO
252 Atk Life Orb Chien-Pao Sacred Sword (see above) vs. 0 HP / 0 Def Quaquaval: 218-257 (70 - 82.6%) -- guaranteed 2HKO
252 Atk Life Orb Chien-Pao Ice Shard vs. -1 0 HP / 0 Def Quaquaval: 73-86 (23.4 - 27.6%) -- 80% chance to 4HKO
252 Atk Quaquaval Aqua Step vs. 0 HP / 4 Def Chien-Pao: 148-175 (49.1 - 58.1%) -- 97.7% chance to 2HKO
252 Atk Expert Belt Quaquaval Close Combat vs. 0 HP / 4 Def Chien-Pao: 1066-1258 (354.1 - 417.9%) -- guaranteed OHKO
Sadly, if Chien-Pao is (crazy enough to use) Life Orb and uses Sacred Sword into Ice Shard, attempting to set up with Aqua Step is not advised, as that attack combination can still KO.
As long as Dragapult isn't Specs/Band/LO/Thunder Wave/Will-o-Wisp, that same Quaquaval set can check Dragapult and snowball in its face:
252 Atk Dragapult Dragon Darts (2 hits) vs. 0 HP / 0 Def Quaquaval: 186-222 (59.8 - 71.3%) -- approx. 2HKO
252 Atk Dragapult Sucker Punch vs. 0 HP / 0 Def Quaquaval: 43-51 (13.8 - 16.3%) -- possible 7HKO
252 SpA Dragapult Draco Meteor vs. 0 HP / 0 SpD Quaquaval: 225-265 (72.3 - 85.2%) -- guaranteed 2HKO
252 Atk Quaquaval Aqua Step vs. 0 HP / 4 Def Dragapult: 78-92 (24.6 - 29%) -- 99.7% chance to 4HKO
252 Atk Expert Belt Quaquaval Ice Spinner vs. 0 HP / 4 Def Dragapult: 250-295 (78.8 - 93%) -- guaranteed 2HKO
...If all else fails, there's checking Kingambit with some KOs under its belt as long as that guy isn't nuts enough to try Aerial Ace or Zen Headbutt (assuming a 1.5x Supreme Overlord multiplier here):
252+ Atk Choice Band Kingambit Kowtow Cleave vs. -1 0 HP / 0 Def Quaquaval: 213-251 (68.4 - 80.7%) -- guaranteed 2HKO
252+ Atk Choice Band Kingambit Sucker Punch vs. -1 0 HP / 0 Def Quaquaval: 175-207 (56.2 - 66.5%) -- guaranteed 2HKO
252+ Atk Choice Band Kingambit Iron Head vs. -1 0 HP / 0 Def Quaquaval: 200-236 (64.3 - 75.8%) -- guaranteed 2HKO
252+ Atk Choice Band Kingambit Brick Break vs. -1 0 HP / 0 Def Quaquaval: 251-296 (80.7 - 95.1%) -- guaranteed 2HKO
252+ Atk Choice Band Kingambit Low Kick (80 BP) vs. -1 0 HP / 0 Def Quaquaval: 267-315 (85.8 - 101.2%) -- 12.5% chance to OHKO
252 Atk Quaquaval Aqua Step vs. 0 HP / 0 Def Kingambit: 106-126 (31 - 36.9%) -- 72.3% chance to 3HKO
252 Atk Expert Belt Quaquaval Close Combat vs. 0 HP / 0 Def Kingambit: 763-898 (223.7 - 263.3%) -- guaranteed OHKO
252 Atk Expert Belt Quaquaval Close Combat vs. 252 HP / 252+ Def Kingambit: 566-667 (140 - 165%) -- guaranteed OHKO
Sadly, you cannot try setting up Aqua Step on this guy, and you pretty much have to Close Combat Kingambit immediately.
This Quaquaval gets forced out by the one guy using (Chilling Water) Toxapex, but Quaquaval gets access to U-turn, so a slightly different set at least gets momentum on Pex.
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I'm personally excited to try Gholdengo in OU, but the sheer number of crazy Ghosts and Darks in the initial meta means that this guy is going to need plenty of support (Dragapult check, Chien-Pao check, Chi-Yu check, Flutter Mane check, Roaring Moon check, Ting-Lu check, Iron Jugulis check, the list goes on...). I think a Substitute set will be among the best in the initial meta as a result, but that Dragapult check had better do work because Pult has Infiltrator.
Gholdengo @ Leftovers
Ability: Good As Gold
EVs: 252 HP / 96 Def / 132 SpD / 28 Spe
Calm Nature
IVs: 0 Atk
Tera: Steel / Fairy
- Shadow Ball
- Make It Rain
- Substitute
- Recover / Thunder Wave / Dazzling Gleam / Focus Blast / Nasty Plot
This set dumps all EVs not needed in the calcs for Speed. Even with no Speed EVs, at least you outspeed max. Speed Adamant Kingambit and Sub in its face, and you also outspeed paralyzed Flutter Mane (in case you think Thunder Wave is better than Sub - Scarf makes both outspeed you).
At the very least, pushing Iron Valiant into using Knock Off (and physical Contrary Enamorus into using Earth Power, and special attacker Enamorus into using Earth Power) and making both fail to OHKO Gholdengo with that with the proper EVs is funny (as long as Iron Valiant doesn't get Sun support to go with that offensive item, that is - also note that Specs Mystical Fire Enamorus can 2HKO Gholdengo with that and avoid the OHKO in return some of the time with this Gholdengo set):
252 Atk Choice Band Iron Valiant Knock Off (97.5 BP) vs. 252 HP / 96 Def Gholdengo: 300-354 (79.3 - 93.6%) -- guaranteed 2HKO after Stealth Rock
0 SpA Gholdengo Flash Cannon vs. 0 HP / 4 SpD Iron Valiant: 332-392 (114.8 - 135.6%) -- guaranteed OHKO (ouch, not even Make It Rain)
252 SpA Choice Specs Enamorus Earth Power vs. 252 HP / 132+ SpD Gholdengo: 260-306 (68.7 - 80.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
0 SpA Gholdengo Make It Rain vs. 0 HP / 4 SpD Enamorus: 396-468 (137 - 161.9%) -- guaranteed OHKO
Actually, scrap that, this Gholdengo wins against Flutter Mane as long as it isn't Specs or has Sun support (you need 252/252+ Calm Gholdengo to survive Specs Flutter Mane Shadow Ball, and even that set has a chance of getting OHKOd after Stealth Rock):
252 SpA Life Orb Flutter Mane Shadow Ball vs. 252 HP / 132+ SpD Gholdengo: 299-354 (79.1 - 93.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
0 SpA Gholdengo Make It Rain vs. 4 HP / 0 SpD Flutter Mane: 254-302 (100.7 - 119.8%) -- guaranteed OHKO