And there have been generals like Washington, who were frequently defeated but still achieved their goals. They weren't the greatest of generals, but do they count as "winners" or "losers"?
Also, a general or army that was top flight in its prime might decay over time to the point where a lesser commander can beat them. I'm not convinced that if we examine their whole careers, that Wellington would come off a better general than Napoleon.
Even if Wellington does come out better for his victory, it doesn't mean that Napoleon -- whatever his faults -- wasn't one of history's greatest commanders.
I don't really disagree with you.
The trouble is in finding some sort of criteria you can apply across places and times. Napoleon and Wellington met on one battlefield, but otherwise were not on the same plane of existence. Napoleon was an Emperor, not just the commander of an army, but the commander of armies and ruler of nations. And he got there through his sword. Wellington was a good general who, along with Blucher, won the Battle of Waterloo (without Blucher's reinforcements, the English would have been ground down to nothing at Waterloo. They won, of course, but the French only broke because the Prussians began to arrive and Napoleon got desperate and launched a desperate gambit which failed. This is not to denigrate the Allied victory at Waterloo, but it is to point out something. Wellington was not the victor at Waterloo. Wellington AND Blucher were the victors at Waterloo. This was one famous battle. Wellington did not drive the agenda of Europe for a decade. Napoleon did.
Perhaps a different criteria yet again we could apply to determine the greatest generals would be charisma: which ones were the true leaders of men in the way that Alexander and Napoleon, and Nelson and Lee clearly were, but Eisenhower and Wellington, great though they were, were not. MacArthur had that quality, but he made a couple of staggering mistakes and got surprised, which reduces him.
If we use those criteria, then the greatest would have to be said to be Cyrus the Great, Alexander, Caesar, Trajan, Aetius, Charlemagne, Napoleon...it's difficult to be objective in modern times.
Of the American World War II generals, MacArthur had the charisma, but he made some real bad mistakes in getting surprised in the Philippines and Korea.
Eisenhower was a master planner and diplomat, and by other criteria I suggested above, truly great. But the standard of charismatic leadership, however, he doesn't quite match up.
Maybe Patton, although it is difficult to say.