In popular culture the Battle of Waterloo stands as the battle that defeated Napoleon. But from what I have read, it seems that even if Napoleon won at Waterloo, it would be impossible for him to win the war with basically all of Europe being united against him. The Battle of Leipzig on the other hand, was both much larger (largest battle prior to WW1) and Napoleons loss at Leipzig led to his exile at Elba and the loss of allies in Italy and Germany, which led to his eventual defeat. Could it, because of this, be said that Napoleon lost the war at Leipzig (or even in Russia in 1812), not at Waterloo?
It would generally be agreed that after Leipzig, Napoleon lost any chance of the initiative. With his armies crushed and many of the Confederation of the Rhine members turning away from Napoleon, he was running out of support and buffer zones. While the campaign doesn't progress farther until 1814, Leipzig is the certain defeat of Napoleon and does herald the defeat of his Empire.
I wouldn't say that Napoleon lost after Russia, he had a chance and he certainly had the numbers to do well against the Allies, but due to his poor use of the Marshals as well as the fact that he never trained any of them in independent command, he had set himself up long before to be defeated.