Carrying over from the heroes to jerks thread in the PF.
Greatest Flag Officer of WWII? Could include General, Admiral or Field Marshall.
I'd have to go with one of the following:
General George Patton. His ability to format a plan of action along with surrounding himself with great leaders that would make it work was key to the African, Sicilian and Western theater. And many of those leaders would go on key roles or even surpass him like Abrams, Truscott and Bradley. And of course he was instrumental in liberating massive amounts of territory and his invasion into Germany. And very likely could have bashed his way deep into Germany in 1944 had Eisenhower thrown the support towards 3rd Army rather than Montgomery for Market-Garden.
The other, and likely as equal, would be Marshall Georgy Zhukov. Between breaking the Axis at Stalingrad and the eventual counteroffensive there and the Battle of Kursk, his was a master at waiting on his opponents to toss themselves onto his defensive works and then going in for the kill when they exhausted themselves. And of course the Eastern Front culminated with the Battle of Berlin in which the Germans finally surrendered.
For Patton's aggressiveness, you had Zhukov's patience. And without the two, I think the war in Europe might have drug on even longer.
Dark horse category certainly goes to Admiral Raymond Spruance. For being tossed into command of a carrier group less than a week prior to the Battle of Midway to the Battle of the Philippine Sea, his grasp of modern naval warfare and the application of carrier tactics were instrumental in retaking the Pacific. And the utter destruction of the Japanese fleet as an effective fighting force. While the Battle of Leyte Gulf was larger, the Japanese had run out of seasoned pilots by then and resorted to kamikaze attacks against the US Fleet. And due in large part to the previous engagement in the Philippine Sea where their naval air had been decimated. Spruance was seen as cautious, but made the right decisions at the right time of when and where to engage. And more often than not, those decisions were spot on allowing lopsided victories on the part of his forces.