More specifically who were they for the Americans in the Pacific and Europe, the British, the Germans, the Japanese and the Russians.
For Germany it was Field Marshal Fedor Von Bock, whose car was strafed by British airplanes and was killed in the last weeks of the war.
For USA it was Lt.General Simon Bolivar Buckner, Jr. in the Pacific, who was killed by Japanese artillery on Okinawa, and Lt.Gen Leslie McNair in Europe, who was killed by friendly fire when American bombers accidentally hit their own lines during operation Cobra.
For the Japanese I'm not sure about the army, but overall highest ranking officer killed was probably Admiral Yamamoto, who was killed when American forces got information of his travel plans on an airplane, and sent fighters to intercept and shot him down.
I'm not sure about the Soviet Union, but no Soviet Marshals were killed, so it's bound to be someone general ranked.
For the UK I believe it was Lt.General William Gott, whose plane was shot down by the Luftwaffe with him onboard.