For use in Anything Goes only:
The Devil (Vivillon) @ Leftovers
Ability: Compound Eyes
EVs: 152 Def / 104 SpA / 252 Spe
IVs: 0 Atk
- Sleep Powder
- Quiver Dance
Because this is Anything Goes, you can spam Sleep Powder because of the complete lack of Sleep Clause. The objective is simple, really. Thanks to Compound Eyes, Sleep Powder's almost as accurate as Spore, and you also outspeed Smeargle leads that are incredibly common in AG. Use Sleep Powder on whatever unsuspecting poor soul begins the match. If you outspeed them, throw up a substitute. If not, use Quiver Dance and then throw up a substitute. Proceed to set up Quiver Dance while your poor opponent spends nearly the entire match asleep. Those defense EVs are required to make sure you don't get immediately blown away on turn 1 since you don't have a Focus Sash. Leftovers are also required so you don't chip yourself to death from Substitute and residual damage. After you've boosted enough, go to town with Hurricane, which has roughly 91% accuracy and does massive damage to everything in the tier not named Stakataka. Works best as a lead so you can Spore a poor unsuspecting Shuckle or uninvested Primal Groudon. You can use it in the late game as a cleaner, I guess, but if stealth rocks are up it might as well be dead. Hurricane is chosen over Bug Buzz for its higher power and ability to hit the uncommon Shedinja.
Vivillon beats anything below 304 speed that isn't running priority, simple as that. Even if it's a slow powerhouse, you can easily have enough turns to throw up a substitute before said attacker mops you up. It also destroys Ditto because Substitute blocks Imposter.
Vivillon isn't without flaws and there's a lot to talk about. Grass types are immune to Sleep Powder, and a bulky one like Arceus-Grass or Ferrothorn can cut its sweep short because Hurricane without boosts is pretty weak. Pokemon faster than it can blow it away on turn 1, rare leads like Forretress and Cloyster are completely immune to Sleep Powder if they're running Overcoat instead of Sturdy/Skill Link, moves and abilities that can ignore substitutes give it troubles, almost every Choice Scarfer in the game outspeeds Vivillon at +1 or even +2, it can't touch Chansey or the multitude of specially bulky Pokemon in the tier, Whirlwind and Roar destroy it, especially with hazards up, Lugia stonewalls it if it's able to pick up opportunities to spam Roost, and because this is AG, just about everything can one shot Vivillon even with +6 special defense. There's more flaws to list, believe me, but if you know what you're doing and you're out to truly piss off the ladder as the enemy's Geomancy boosted Xerneas or Mega Rayquaza dies a slow, pitiful death to National Dex #666, you can have quite some fun.
You can piss off your friends with the set, too, but this doesn't work in lower tiers because of Sleep Clause.