Mega Charizard Y + Mega Floette Big Six

Big Six is the best team in Pokémon Champions Regulation M-B, and the six pieces work together well enough that almost any pair can lead. Mega Floette's Fairy Aura boosts Whimsicott's Moonblast as well as its own, so Whimsicott hits like a Life Orb attacker while still holding the Focus Sash. Mega Charizard Y is the other half of the mixup. Whimsicott and Basculegion is also a solid lead, but thanks to last respect is often better saved for the late game, so careful when lead with him. Against Toxapex, keep Basculegion and Kingambit alive to close. The mirror is rock paper scissors, and it is the matchup that beats this team. Careful for Charizard with Ancient Power, known variation, but in general lead Whimsicott with Garchomp if you read Charizard, with Floette if you read Garchomp, and with Charizard if you fear their Floette.