After trawling around the internet for a while, I have found that people reckon Napoleon's best chance for victory in Russia in 1812 was at Borodino, where he needs to get a decisive one instead of the Pyrrhic one of OTL. The only problem I have found with this is that it is too late in the campaign: Moscow was about to be burned, Napoleon's army was disintegrating and he was quite deep in enemy territory by then.
The same does not seem to hold true for the battle of Vitebsk (26-27 July 1812). This was fought when Napoleon's objective was still to quarter in Smolensk for the winter, and was close enough to friendly territory (Poland) that supply wasn't too big an issue.
In OTL, the Russians primarily fought a rearguard action that delayed the Grande Armee enough that the Russians could escape across the river towards Smolensk, while Napoleon never brought up all of his forces to fight them. In an ATL, would the forces he had available on the 26th or 27th be enough to rout the Russians, and is a decisive enough victory possible that will allow him to conclude the Russian campaign (say, territory stays as is, but Russia goes back to the Continental System and promises to leave Napoleon alone to fight in Spain).
- BNC