Marshal Louis Davout. If he had actually been the guy in charge, Western history probably would have turned out very differently.
1) The French army of 1814-1815 was not nearly the same terrifying military that crushed the coalitions in 1804-1805. Most military historians, including David Chandler, admit that the French army had significantly deteriorated by 1807, which is before Portugal, Spain, and of course, Russia. Conscription by 1810 had been greatly expanded, and by 1814 included 16 and 60-year-olds.
2) Napoleon's officer corps was shattered. His finest commander, including Murat (his finest cavalry commander), were already gone, many killed in Spain and Russia. At Borodino alone, Napoleon lost 13 generals killed!!! His chief of staff, Berthier, known for writing the most precise of orders, died falling off a house the year before Waterloo, and Napoleon's own pitifully worded instructions to Marshal Grouchy, to "stay at Blucher's back" sealed his doom by depriving Napoleon of 33,000 men in the heat of battle. Berthier never would have allowed such sloppy orders to go out.
3) The aura of the French military had been destroyed in Spain and Portugal. The confidence that they could beat Wellington alone, let alone in concert with other great powers, was not there. After Russia, it was totally shaken. The only ones not afraid were those too young to have experienced battle.
4) Absolutely Napoleon was to blame for "delaying" the attack at Waterloo until the afternoon---but that was precisely because the rain had soaked the ground turning it to mud. Duout warned Napoleon that he would not be able to move the artillery as fast as normal, depriving Napoleon of his typical ability to focus fire on specific parts of the British line.
5) Wellington, though having fewer dependable troops, nevertheless conserved his manpower by hiding them behind the ridges. This had the effect of not only preserving them from artillery fire, but in stunning and shocking both Ney's cavalry (which charged over the ridges into squares) and the subsequent "Old Guard" infantry advance, which again was staggered by the previously undetected reserves.
The fact is, even without Blucher arriving on the flank, Wellington had beaten Napoleon by the end of the day. The Old Guard was crushed, much of the French army was running, and Grouchy, with the only "fresh" reserves, was neither fresh---having marched for two days---nor able to link up with Napoleon because he was separated by Blucher's army.
Finally, Napoleon himself was assisted from the field right at the crucial moment due to his stomach cancer. There was nothing he, nor any other Frenchman, could have done to win that battle after Ney's cavalry charge.
Actually Berthier died just a few days before Waterloo, watching allied troops from a window in his home in Germany.
Fell? Pushed? Jumped? No one knows. But he didn’t join Napoleon for the Hundred Days, so apparently he had seen enough of war.
Marshal Louis Davout. If he had actually been the guy in charge, Western history probably would have turned out very differently.