IMo opinion, the most important crucible in assessing the greatest commanders, is whether they won or not. i know that circumstance are often beyond the control of the commander, but thats just the point, sometimes you have to know when your beat.
Mannerheim was perhaps one of the greatest in this respect. In 1939 he knew that Finland could not beat the USSR, and he told his government as much. They ignored him (for political necessity), and Mannerheim then went on to deliver a series of defensive victories that ensured the survival of this little country.
You cannot lay the same praise at the feet of Napoleon, I believe he was swept off his feet by his own propaganda, and believed he could overrun the whole of Europe. By this reason alone, I believe he must be excluded as the "greatest" commander.
Guderian has an undoubted brilliance, and is one of my personal favourites, but whilst his theories were revolutionary, they eventually became dated, and Guderian never updated his theories to reflect that change. In 1939-41 blitzkrieg embraced two clear elements.....concentration of effort at the schwerepunkt (point of decision...sorry about my terrible german spelling guys) and secondly the employment of the "all arms concept". By 1943, the first mentioned element of blitzkrieg had been circumvented by the Russians and the to a lesser extent by the Allies. Zhukovs "symphany's" worked on the weaknesses of Guderians theories. He would run up and down the front with a series of broad front offensives, that rendered the Germans concentrated armoured reserves located behind the front far less effective than it actually could have been. Too often, and too late, the german armour would arrive on the scene, only to find its Infantry virtually destroyed, along a hopelessly long section of the front, with no hope of the armour being able to plug the breach. The Germans would predictably strip the flanks of this breach to plug the hole torn into the line by the Soviet offensive, the Soviet offensive would continue until it ran out of petrol, men, and ammunition (rather too late IMO, but Soviet C&C was so poor that this was the only way they could do it). In the meantime, Zhukov would open up a new offensive elsewhere, where the front had been weakened by the germans, and so repeat the process allover again.
In this way, Guderians theories, which emphasised the need for concentration, so as to ensure superiority of numbers, at the decisive point, actually worked against the decisive battle. Guderain never understood that the "decisive point" under the new Soviet system, was everywhere, and anywhere....to better counter this, he had to accept inferiorrity of numbers, and have the armour on hand, at the point of breakthrough, immediately...