Since PokeBH seems to be the most popular OMM nowadays I'm gonna share the worst sets I've cooked up. (Click the gifs to see the sets)

I wanted to abuse Magnet Pull in the dumbest ways possible. Unfortunately it got banned before I could use this, but I'm gonna explain this anyway. If your opponent was foolish enough to bring in their Imposter on Magnezone, they would see you only have 3 moves, 2 of which can't be clicked unless they have a berry too. They would be unable to switch because of Magnet Pull, leaving them with no choice but to go boom. Counterplay includes
Shed Shell Imposters, Imposterless teams, steel types, poison types, Knock Off, Trick, Taunt, Ice Scales, ground type moves, and probably a lot more.

Trap the opponent, give it Magic Bounce, boost your attack, then click Strength Sap to force them to faint from Liquid Ooze. After 2 attack boosts, Strength Sap will KO everything except Blissey, Chansey, Guzzlord, Regidrago, Wobbuffet, Cetitan, and Wailord. Counterplay includes normal types, Good as Gold, anything with decent offensive stats, Taunt, Covert Cloak, and more.

Truly skilled trainers should win with their favourites,
so I guess I'm not very skilled. Unfortunately The Scorb has a lower attack stat than

this thing, so I have to use the fully evolved form. It gets 116 attack and STAB on every move and there are loads of strong physical moves it can use, like V-create Glacial Lance, Headlong Rush, Bolt Strike, Close Combat, Sunsteel Strike (or Gigaton Hammer, but it doesn't work that well with band), and more, which hit quite hard when paired with Adaptability:
252 Atk Choice Band Adaptability Cinderace Sunsteel Strike vs. 252 HP / 252+ Def Audino-Mega: 368-436 (89.7 - 106.3%) -- guaranteed OHKO after Stealth Rock
252 Atk Choice Band Adaptability Cinderace Glacial Lance vs. 252 HP / 252 Def Blissey: 716-844 (100.2 - 118.2%) -- guaranteed OHKO
252 Atk Choice Band Adaptability Cinderace Glacial Lance vs. 252 HP / 252 Def Eternatus: 588-696 (121.4 - 143.8%) -- guaranteed OHKO
252 Atk Choice Band Adaptability Cinderace Glacial Lance vs. 252 HP / 252 Def Ting-Lu: 492-580 (95.7 - 112.8%) -- guaranteed OHKO after Stealth Rock
252 Atk Choice Band Adaptability Cinderace Bolt Strike vs. 252 HP / 252 Def Ho-Oh: 664-784 (159.6 - 188.4%) -- guaranteed OHKO
252 Atk Choice Band Adaptability Cinderace Bolt Strike vs. 252 HP / 252+ Def Dondozo: 516-608 (102.3 - 120.6%) -- guaranteed OHKO
252 Atk Choice Band Adaptability Cinderace Headlong Rush vs. 252 HP / 252+ Def Toxapex: 384-456 (126.3 - 150%) -- guaranteed OHKO
252 Atk Choice Band Adaptability Cinderace Headlong Rush vs. 252 HP / 252 Def Tyranitar-Mega: 428-508 (105.9 - 125.7%) -- guaranteed OHKO
252 Atk Choice Band Adaptability Cinderace Glacial Lance vs. 252 HP / 252+ Def Fur Coat Gliscor: 448-536 (126.5 - 151.4%) -- guaranteed OHKO
Also gets Blaze which turns V-create into a nuke if it's low
+1 252 Atk Choice Band Adaptability Cinderace V-create vs. 252 HP / 252+ Def Fur Coat Steelix-Mega: 316-372 (89.2 - 105%) -- 50% chance to OHKO after Stealth Rock
+1 252 Atk Choice Band Adaptability Cinderace V-create vs. 252 HP / 252+ Def Arceus: 516-608 (116.2 - 136.9%) -- guaranteed OHKO
+1 252 Atk Choice Band Adaptability Cinderace V-create vs. 252 HP / 252+ Def Toxapex: 216-255 (71 - 83.8%) -- guaranteed 2HKO after Stealth Rock
Ok maybe this is actually viable (you could also run adamant but then Arceus always outspeeds it)

What in the world is this abomination? Well, Klutz lets you gift your opponent an Assault Vest even though it's still 2 days until Christmas, and in return, you can potentially get a Rocky Helmet or Covert Cloak that will have stopped this from working. No Guard ensures you can paralyse your opponent with Zap Cannon and trap it with Magma Storm. It also means Population Bomb will always hit 10 times, and Stench means you have a 65.13215599% chance to flinch them. Either this is a genius set or (probably) absolute trash.