Well I'd have to say Kasumi-chan/Misty,mainly because Hikari and Haruka basically focused on Contests (and not battles)..Iris I believe is getting there with her Kibago and her growing team no I'm not saying this because she's a gym leader (referring to Kasumi/Misty.)..Episodes with Kasumi-chan mainly focused on battles and such with no contests..and is now with Dento and Iris (well so far) anyway..so I think Kasumi is most likely the best at battling..but if they each were to have a battle then that would be something different.

I have to say Misty. I am saying this because these are plain Pokemon battles. When a coordinator goes into a normal battle, it must be difficult since they usually command their Pokemon to perform moves that not only reduce HP of the competitor, but also look good. As for Misty, who often battled regular battles (and sometimes pushed Ash away so she could battle instead lol), she must be better because she would only ever focuse on defense and reducing HP.

As for Iris, well, she still has a few seasons to grow stronger

So much agree with both of you. And yeah, totally agree with Espeongirl360 that the Pokemon moves they command are not only to reduce the HP, but also look good at the same time.
For May, if she does plain battles, she will be really good at it, as far i know May, she is really a tough co-ordinator, she always used to do better at the battle rounds than the preliminary rounds, means that she is a more of a power co-ordinator.
And for Misty, she is really a Good Pokemon Trainer, she always don't use to battle but that don't mean she should be taken less!
Training her Gyarados to learn flamethrower is much impressive. She is really a good trainer and a good Gym leader.
I would have liked to pick both May and Misty, but i will go with Misty as in plain Pokemon battles she is better than the others! and also agree with McHopnPop that if they were to battle then it would be something different.
And i think, Misty will have a good chance.

I say Misty. She has most experience above all.

misty...bcuz shes a gym leader therefore has experience nd gained experience while travelling wit ash...may is average but is meant 4 contests...dawn is a average battler nd does pretty well contest wise...iris...may or may not b a gym leader nd her true experience level is unknown...

This is one of the most pointless polls ever. You know that 50% of the people in this poll (Dawn & May) are coordinators. NOT pokemon battles. So why not just Iris and Misty? Oh wait Iris wants to be a dragon master (it could most likely mean that people to talk to dragon types). So Misty, the only pokemon girl who does pokemon battles for a living.

So co-ordinators don't battle? Plain battles or contest battles i don't think it creates any sort of unfairness, after all its about the strategy seeing how Drew battled with Ash and defeating him, it was totally fair & square, Fantina the gym leader is also a co-ordinator and Dawn participating in the world tournament is another boo in yo face. Therefore, your point is invalid. ;)