My initial thought was kasserine pass in the Africa campaign or market garden. It seems like we don't discuss defeat to Germany much in the US.
I am preparing for a zinger if the Americans get trounced by Germany tomorrow in the World Cup.
Kasserine pass was probably the most shocking because it was the americans first major encounter with the germans but men and material loss wise id say the battle of the bulge did more damage and had a greater impact on the western allies as a whole.