The sceptic makes no assertion, he's asking you to prove your claim. No more, no less. Asking for proof doesn't mean asserting the opposite and even if you cannot prove your claim, it doesn't mean that your claim is incorrect. It just means you couldn't prove it. And sometimes one should just admit to be unable to prove a certain claim, even if it is correct, and change scope to something provable. Especially when the claim is unprovable to begin with, like "being the best" usually is.