My money is on Julius Caesar. Never defeated which is more than Napoleon can say.
And his generalship at Alesia where he build a double wall to keep the Gauls on the mountaintop besieged while fighting off a numerically superior force on the outer wall at the same time has NEVER been done before or since. Not to mention, he personally rode to the rescue his troops at a point in the battle where his legions were endanger of breaking.
I'd rank Julius above Nappie, and not just because he reads better; but Napoleon was best of his era, of that I wasn't in doubt before reading about this study. OTOH, I might rank Pompey the Great above Caesar, even though Caesar ultimately defeated Pompey -- both however added plenty of territory to the Roman Empire, and both were treated like dirt by the corrupt, lying jackasses of the Roman Senate.
And I know it is more of an engineering marvel of its day (or even today) but the bridge his army built over the Rhine in what - a week? I watched a documentary on that wherein they really could not explain how the Romans were able to do it in the time frame they did. Quite amazing. But it does point to leadership and motivation.