Listen, I'd like to join the bandwagon and say Austerlitz, but I got to say this:
Although it contibuted to Napoleon's defeat, the abandoning and burning of Moscow was, by all means, the most catastrophic defeat for the allies, or at least for Russia before finally taking advantage of it.
Let's put it this way:
It was a defeat for Russia
It contributed heavily towards Napoleon's defeat.
Nonetheless, the Russians lost their [spiritual] capital (Petersburg was the political capital). In the sense that they lost it, and burned it to ashes, it marked to the world that conventional warfare could not, and would not bring Napoleon down.