Germany conquered most of Europe. In the end, Germany began to decline due to Hitler's mistakes. However German tanks were some of the best in WWII. German tactics outmatched those of any other country. Germany had one of the largest armies in the world. I would say that Germany had the strongest military.
In mid-1940, Germany occupied Poland, Denmark, France, Belgium, Holland, Austria, Norway, Greece, Yugoslavia, and had countless troops in the Balkans massed along the Russian border. Germany had the best tactics in the entire war, however Blitzkrieg is great for offensive preemptive strike operations, they aren't so effective for defensive maneuvers. In 1940, the United States didn't have the largest military, they didn't even have the largest until 1960s-1970s-1980s. The Soviet Union had the largest on stand army throughout the Second World War and the first half of the Cold War. Millions of troops, thousands of tanks, countless aircraft, and hundreds of ships. The Germans never produced over 10,000 tanks, the Soviets had more than that in 1930's-1940's. The Germans destroyed most of the Soviet Airforce in the first weeks of Operation Barbarossa, but it picked back up in 1943. The largest military from the mid-1930's was Germany but after the Molotov-Ribbentrop agreement in 1939, Stalin knew Hitler would attack, he just didn't know when, and when he did, Stalin was so surprised it was so soon. Stalin had been amassing troops for weeks but Hitler had 4 million troops, 4,000 tanks, and 8,000 aircraft poised to invade in just June of 1941. After the defeat at Stalingrad, Kursk, and Leningrad, the war had turned in Stalins favor. Hitler couldn't have won by then. Hitler was a great politician but a terrible military strategist, this is what led to the failure; Hitler began dictating the operations of the Heer, Kreigsmarine, Luftwaffe, and Schutstaffel.
Look, Germany had a ridiculous army back then, don't get me wrong. However, there was a reason that Germany began to fall. Although it had some superior tech over the US, they simply could not match the funds and production of the US. This is why we crushed Japan in the Pacific. We started out with only a few more aircraft carriers then them, then we just made a ton more.
Although Germany had the greatest standing military in 1940-41, that is only because the US had not yet begun to really recruit or begin to get its army together. The US had the far greater military capacity, yet Germany had the greatest military then.
The Red Army was the strongest and most technically advanced army at the time. Stalin was planning a surprise attack on Germany and had massed men, artillery, tanks, and aircraft as close as 800 meters from the border with Germany.
Hitler did NOT want war with Russia. It was only after the U.S.S.R. Had violated the Peace Treaty and their build up became apparent that the Germans attacked Russia. When the German Army DID attack Russia, they utilized 750,000 HORSES to move forward!
The Germans were STUNNED by the technology they captured. The only reason that the German Army was successful in the first year is because 1) The Red Army was preparing for an OFFENSIVE war against Germany, which is a very weak position DEFENSIVELY and 2) There was so much excellent Russian equipment massed at the border that was captured, then used AGAINST the Red Army.
Viktor Suvorov, a Soviet Intelligence Officer who defected in 1978 to the United States has written the book "Icebreaker" that successfully demonstrates this. Also, his book "The Chief Culprit: Stalin's Grand Design to Start WW2 takes the idea further.
This is all very "Politically Incorrect" today, but that is changing. The are too many facts coming out now. Too many people have an interest in keeping Hitler as the guy who started the war when it is now seen that he was the one who tried to avoid war.