Based on stats, moves, etc. alone, I'd say that Vivillon is the best, simply because of its movepool. Powder is a great, although situational, signature move, and it puts the opponent on their feet, trying to predict what move you'll use. It also gets access to Quiver Dance, which boosts all of its important stats. Then its coverage is fairly strong, with access to Grass, Psychic, and Fairy moves, along with a relatively strong support viability, with moves such as Aromatherapy that could give it a possible role in utility.