Misty is at a distinct disadvantage really since Dragon types resist Water and Iris has Emolga, May has Venosaur and Dawn has Pachirisu. She has some powerful pokemon ie. Starmie and Gyarados but I higly doubt she'd come off on top.
May has a very well balanced team and some very powerful pokemon with Blaziken, Glaceon and Venosaur on her team. So she'd have a very good chance of winning. Although the others do have various methods of countering her.
Dawn has a very varied team and powerhouses Mamoswine and Togekiss (as well as a Pikachuesque Piplup) although she may not be as varied as May I'd give her a fairly good chance. As seen in the anime she is better at coming up with strategies and techniques than the others though.
Iris, like Misty, is at a rather serious disadvantage since two out of three of the others have an Ice type and Misty's pokemon can pretty much all learn ice moves. She has powerful pokemon on her team with Excadrill and a pseudo-legendary (Dragonite) but there are some behavioural issues on her team.
So to sum it up either May or Dawn would be most likely to win in my opinion. Iris would have a small chance due to the sheer power Dragonite offers her but quite frankly her crippling weakness to Ice types and her pokemon's disobedience would put her in a fairly bad position. Misty would have the smallest upon smallest of chances. The three others all have some method of countering water type moves and seem to have generally more powerful pokemon. I'd give her a shot at beating Iris since Excadrill is weak to Water and her whole team is weak to ice (which water types can almost all learn for memory).