Ike, Ike, all day long, a magnificent tour de force in commanding the forces of the allies. I read his book "Crusade in Europe" - his insights, his reasoning, his unswerving dedication, his brilliance at getting the best out of his generals. Standing up to the politicians, sheer pragmatic tenacity, wielding the most power ever wielded without the tiniest corruption. We all literally have this man to thank for our freedom. WWII is full of heroes but Eisenhower gave them their stage.
I'd go for Alexander too, for the reasons already stated. Ike did well (last US president without political experience, so a model for Trump) but he learnt to distrust the British during the war and - wisely - hung them out to dry over Suez.
1. Heinz Guderian
From the 1920s he advocated the creation of tank divisions and later became one of the leading generals in the German campaigns in Poland, France and the USSR. Had Hitler taken his advice in 1941, Moscow would probably have fallen.
2. Frederick the Great
In addition to being an able military commander, he was a wise political leader who enlarged Prussia and helped pave the way for German unification a century later.
Thinking again, von Moltke would agree with moinonplus ^ that Frederick the Great was supreme, even when Napoleon visited Frederick's grave he ordered his officers to remove their hats.
However, some of the strategies defined by von Moltke were still used by Rommel in WW2.