Open for debate. You'd be surprized how sorry the U.S power allone was in 1939, be it in on the ground or the air.
Surely there were numbers, - the western allies had much more "so called" power than the Germans in the beginning days of WW2, but the Germans showed a different muscle, - a combination of plans and forces, and promptly delivered spanking lessons left and right.
I stick to my statement of them being the strongest power alone, - that criteria means complete seperation of course.
Strongest in the air : Yes. Second would be the British
Strongest on the seas: Well, deadly, but the cup goes to the Brits.
Strongest on land: Practically yes.
Strongest on resources: Before Barbarossa probably so.
So, all in all, the strongest "single" force. Swamping up most of the mainland of Europe made the Germans a mighty power which it took several years of combined efforts to smack down.