I don't really like Kakashi but I think he would quite easily win this one. He's got a big old speed advantage and with his sharingan and raikiri it would probably play out something like this:
Natsu tries using his flame breath but Kakashi keeps dodging. Natsu gets pissed off and finally his flame breath connects but its only a water clone. Natsu looks confused and realises too late that Kakashi is standing behind him. Kakashi pierces him through the stomach with Raikiri. Game over.
@Haofan123: I honestly think people rate Kamui too highly. It's risks outweigh its benefits and there's a reason Kakashi hardly ever uses it. For one it's not the most accurate of jutsu, yeah he stopped a nail with it but that nail was moving in a nice straight line. It's only really useful for stopping big objects and objects moving in a single trajectory and it's drawback is huge, when he uses it he's pretty much out of the fight.
I really don't think he would be likely to use it against Natsu, when he could probably beat him with Raikiri and against an opponent who was faster than Kakashi I doubt he would use it anyway because he'd probably miss.