Its certainly a myth that the German army had some inherent superiority over the other armies in World War Two. Germany's initial "superiority" is the result of being better prepared and very good commanders who knew how to best use the forces they were given. The walkover campaigns in Poland, France, and the opening stages of Operation Barbarossa helped foster this idea of the unbeatable Wehrmacht that steamrolled any army put in front of it. The defeat of France was specifically key in creating this image, because France was the traditional enemy of Germany that had given it so much trouble in WW1. But there were reasons that the German Army so easily defeated the French and Soviet armies. It wasn't that the German military was "great" but rather that their enemies were unprepared to deal with the German military threat.

Lets start with France, French rearmament had been a topic since the early 1930's, but it didn't become a major priority until after Germany re-militarized the Rhineland in 1936. This meant that in key areas the French were significantly outnumbered by the Germans. Key areas including AA-Guns, and Aircraft. This was to play a key role in the French defeat when German Stukas harried allied lines, and when Allied Bombers proved unable to retaliate because the Germans held total air superiority. But the French did outnumber and outclass the Germans in tank production and artillery production.

The bigger disparities were in doctrine, going back to this point

very good commanders who knew how to best use the forces they were given

The German commanders and soldiers were battle hardened and had been able to test their tactics and strategies in the Polish campaign. They had perfected the strategy of using rapid armoured spearheads combined with support aircraft to overwhelm their opponents. The German overall strategy, the "Manstein Plan", was nothing short of brilliant. Meanwhile the French were still committed to using their tanks in support of their infantry, the French had begun to build independent tank divisions, but they only had three with a fourth being built. The vast majority of French tanks were still spread out where they were of little use.

But again I want to stress that it wasn't that the German army wasn't unbeatable, but rather that the French weren't prepared for this style of warfare, given more time they may have been able to counter the Germans and win.

As for the Soviets, they were just as unprepared. The Soviet's had once been leaders in military innovation. But Stalin's purges had gotten rid of Soviet military officials who wanted to modernize and instead replaced them with Conservatives like Marshal Kulik who were not in favour of independent tank divisions like the Germans had. So we have a situation where even though the Soviets have more tanks, there numerical superiority was nullified by incorrect use of the tanks. The aircraft situation also favoured Germany. Germany's aircraft while in some ways of lesser quality (though that's extremely arguable) and of lesser number were able to destroy their Soviet counterparts easily due to superior leadership (and a heavy dose of surprise). Soviet forces were also often hamstrung by logistics and by political considerations which made it easier for Germany to pull off those massive encirclement of 1941. But as we know, when the Red Army got its act together, it proved more than capable of defeating the German Army.

As I conclude my rambling, I want to add one last point. This myth all though its origins can be found in the rapid campaigns of 1939-1941, really started to gain traction at the end of 1941. Why? Simply put because that is when Operation Barbarossa failed and when Germany's second "stab in the back myth emerged", this particular iteration being that if Germany's generals had just been allowed to fight without political interference form Hitler, the German army would have won. Hitler's generals after the war helped spread the idea of this invincible fighting force that was "betrayed" by Hitler's incompetence.

Sources:

The Fall of France by Julian Jackson

When Titans Clashed by David Glantz