Light Ball

Pikachu's signature item. Held by anything else, the Light Ball does nothing — the boost is gated on the holder being Pikachu specifically, so even Pichu and Raichu don't qualify. The simultaneous ×2 on both offensive stats pushes the otherwise-frail Mouse Pokémon into OU-relevant power tiers, which is why it's banned in most format pools alongside Mega Stones.