Ed and Charles hit some of the points.   The US auto industry put out massive numbers of tanks, which after a few bugs were worked out became very reliable machines, with a huge supply of standardized repair parts to back them up.  There was also a vastly higher percentage of Americans who grew up around automobiles, trucks, tractors, etc. compared to most of the European countries, AND spent the Depression years learning how to keep those things running.   

The Russians had the T-34 and a couple heavier units that were probably the best all-around armor designs of the war, but inferior manufacturing and poor maintenance in the field hampered them all along.   The Germans had some armor that was very capable, but also more hand-built rather than standardized on a mass production basis that put out a great supply chain.   And for instance the Tiger was a world-beater, but transporting them to the Eastern front was a terrible problem for the railroads, and those heavy tanks had mobility problems with mud and primitive river bridges.

So yes, in the right circumstances, the best German tank could sit in one spot and kill multiple Shermans or whatever.   But a squadron of Shermans with ability to maneuver would still win.   And, really, the whole point of armor is to get the rifleman onto terrain that he can hold.