Germany, without a doubt. Thankfully it was offset by the political handicaps and control that Hitler exerted over his high command, demanding political maneuvers rather than listening to his best Generals. The overall commanders of the German Military were more political appointees, not competent soldiers. Along with a German technological and training advantage, they -should- have won the Second World war. Thank god for politics.
British Generals were adequate, but they made some exceptional blunders, and their primary actions were to react to German or Japanese assaults. (Need I mention Market-Garden? More Costly operation than D-Day. Or the Fall of Singapore?)
American Generals were good, but I think it mainly comes from a grinding perseverance rather than a true Strategic ability. North Africa/Italy and France were mainly grinding campaigns with one objective, and they also made their share of mistakes. (The Ardennes , Anzio, Kassarine Pass, The Philippines to name a couple.)
Russian Generals were talented, and they were exceptionally good Strategic planners, although their actions were limited by Stalin as much as Germany's were limited by Hitler. Not a step back, hold your ground, attack at all costs...not exactly a good way of utilizing your nation's advantages; space, space, space and more space.
France had some Excellent Generals, but they didn't get to operate or even fight for the most part. De'Gaulle had some excellent ideas that might have countered the German Blitzkrieg, but he didn't get the chance to implement them because of the French Government's political decisions.
In the end, I don't think you can really claim that the war was decided by Generals or by armies, once Japan brought America into the war in the Pacific, the war was decided, just as when Germany brought Russia into the war in Europe, it was decided. Industry and resources won the war for the Allies. There were many opportunities for the Germans to win the war in Europe with a negotiated peace (Invade England, or even easier, destroy the BEF at Dunkirk and force a a negotiated end by England.) And the opportunity for the United States to completely avoid a war in the Pacific. (The Japanese were willing to negotiate a withdraw from China after the Oil and Steel embargo so long as token face saving measures could be taken.)
