Marshal Louis Davout. If he had actually been the guy in charge, Western history probably would have turned out very differently.
I don’t think even he could have saved Waterloo. IF Blucher had actually been crushed earlier, instead of just bloodied, yes, Wellington might have lost. But there was still a massive Russian army out there, and the rest of Europe could mobilize again. Moreover, Davout as a general might have been terrific, but as a charismatic leader, Napoleon had everyone beat.