As I see it, the single most persuasive secular argument for the authenticity of the founding events of the Restoration isn't actually a single argument or line of evidence at all. It's the fact that no counterexplanation yet proposed for those events accounts for all the relevant data nearly as well as Joseph Smith's own story does.
If his claims aren't true, the fundamental question to be asked regarding Joseph is whether he thought they were or knew they weren't. In other words, was he in some sense "crazy" or was he consciously deceptive?
Let's take those two options in that order.