WaterShuriken
"I..I..Iron Tail, Pikachu."
Well, Serena could win if it's a situation similar to to the original Wallace Cup if Serena's Braixen beats Dawn's newly captured Pokémon.Honestly I wouldn't mind a Serena win, as long as it adds something to her performing career.
However if all three girls show up, May should win as a new Top Coordinator. Dawn made it further in her GF but she didn't a Solidad or Robert where it's obvious they were seasoned vets. Everyone in the Sinnoh GF was entering for the first time.