For an armor enthusiast like me it's got to be "Unternehmen Zitadelle". This was where the Germans lost the initiative in the east for good. While there would be numerous offensive operations in the east including larger ones such as Operation Conrad I, II, III in Hungary, the Germans were on the defensive from here until the end of the war.
The mix of german armor is quite interesting including Panzer III, IV, V (Panther), VI (Tiger), Ferdinand, Nashorn, StuG, Brumbar, Marder, captured T-34 etc.