Misty's a gym leader, but is monotype, so that seriously hinders her chances of beating May's Beautifly and Venusaur. Although Misty's Gyarados knows Flamethrower.
Dawn also has two powerhouses in Togekiss and Mamoswine and those two would be tough for the others to deal with. Iris has Dragonite and Excadrill, but although both are extremely powerful, they have each had severe listening problems, which shows how Iris is still growing as a trainer. Her Pokemon don't always listen to her.
So let's say these were the teams used, based on each character's favoritism:
Misty: Gyarados, Politoed, Corsola
May: Blaziken, Venusaur, Glaceon
Dawn: Piplup, Togekiss, Mamoswine
Iris: Excadrill, Emolga, Dragonite
Iris could get owned by May's Glaceon and Dawn's Mamoswine, and it wouldn't surprise me if Misty had an ice attack on her Politoed by now, to deal with Grass-types. I honestly think Iris doesn't have a change.
Misty is a gym leader and has undergone intense training, so I wouldn't count her out. It's really difficult to call - all of them have their pros and cons, and it might come down to luck in a round-robin tournament...