I'll be voting BAN
Chien Pao is the most threatning mon in OU, he is the second fastest pokémon of the tier, second only to dragapult in speed (that dies to a +1 (CB) Ice Shard
252 Atk Choice Band Sword of Ruin Chien-Pao Ice Shard vs. 0 HP / 0 Def Dragapult: 314-372 (99 - 117.3%) -- 93.8% chance to OHKO
Possible damage amounts: (314, 318, 320, 326, 330, 332, 336, 342, 344, 348, 350, 356, 360, 362, 366, 372)
So, it's very hard to revenge kill it without a scarf or a super effective priority mon (As Pawmot, Slither Wing or Breloom [If chien pao is not locked in Ice Shard] Most of people say that his best counter is Dondozo, but
252 Atk Choice Band Sword of Ruin Tera Dark Chien-Pao Crunch vs. 252 HP / 252+ Def Dondozo: 214-254 (42.4 - 50.3%) -- 92.6% chance to 2HKO (After Rocks or 1 Layer of spikes)
People talk about Tauros-Water and Tauros-Blaze, yes, they're pretty good counters, but they lack recovery, depending of leftovers to keep health in long matches, when they're not holding lefties, they hold HdB to switch in with safety in Hazard Stack teams, however, they lose their recovery,
the 2 hardest counters of Chien Pao aren't even OU
There are some calcs:
252 Atk Choice Band Sword of Ruin Tera Dark Chien-Pao Crunch vs. 252 HP / 252+ Def Garganacl: 198-234 (49 - 57.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 Atk Choice Band Sword of Ruin Tera Dark Chien-Pao Crunch vs. 252 HP / 252+ Def Rotom-Wash: 226-268 (74.3 - 88.1%) -- 6.3% chance to OHKO after Stealth Rock
252 Atk Choice Band Sword of Ruin Tera Dark Chien-Pao Crunch vs. 252 HP / 252+ Def Toxapex: 176-208 (57.8 - 68.4%) -- guaranteed 2HKO after Stealth Rock and Black Sludge recovery
252 Atk Choice Band Sword of Ruin Tera Dark Chien-Pao Sacred Sword vs. 200 HP / 180+ Def Quaquaval: 164-194 (45.4 - 53.7%) -- 41% chance to 2HKO after Stealth Rock and Leftovers recovery (Guaranteed with 1 layer of spikes)
252 Atk Choice Band Sword of Ruin Tera Dark Chien-Pao Ice Shard vs. 0 HP / 0 Def Breloom: 300-354 (114.9 - 135.6%) -- guaranteed OHKO
Possible damage amounts: (300, 302, 306, 308, 314, 318, 320, 324, 326, 330, 336, 338, 342, 344, 348, 354)
252 Atk Choice Band Sword of Ruin Tera Dark Chien-Pao Ice Shard vs. 0 HP / 0 Def Meowscarada: 332-392 (113.3 - 133.7%) -- guaranteed OHKO
252 Atk Choice Band Sword of Ruin Tera Dark Chien-Pao Ice Shard vs. 0 HP / 0 Def Roaring Moon: 330-390 (94 - 111.1%) -- guaranteed OHKO after Stealth Rock
Let's talk about HdB, HdB Pao is pretty versatile and suffer of Magearna Syndrome (Depending of his set, it breaks your entire team)
there are so many variations as Standard AoA Pao (Smogon set) Tera Electric Pao (SD Ice Spinner Sucker Punch TeraBlast) (Bolt Beam is one of greatest Coverages of all time, it 2hkoes Dondozo after rocks, OHKOes Pex and Corv with +2 Tera Blast, it can be used defensively to survive some hits and Dance or kill some pokémon, as Iron Head Kingambit, Choice Locked Gholdengo, and Bpunch Scizor, Tera Ghost Pao (Same set as AoA, but with tera Ghost) Tera Ghost makes it harder to revenge kill, cuz it gets immunity to Extreme Speed, Mach Punch, and resistance to First Impression
Tera Ghost Pao can dance in front of things as BPress Corviknight and can surprise Choice Locked Iron Valiant and Breloom, Tera Ground Pao can break some checks as Tauros-Paldea-Fire, Garganacl, Kingambit and Toxapex after a Swords Dance
There are more sets of HdB and SD Pao, but what I'm trying to say is, his speed and priorities makes him a killing machine, very hard to revenge kill due his high speed, priorities and insane dmg because of Sword Of Ruin, being a high threat in most of matches, CB Pao is bizarrely strong, almost every game when it gets momentum, Chien-Pao just forces the opponnent to run a 50/50 between switch to a Dark resistance as Great Tusk or switch into a Ice resistance, as Rotom-Wash and Toxapex.
Chien Pao is pretty strong to OverUsed, this is why i'll be voting BAN (Sorry for my bad english, I'm learning it)