I have read that the 8th Air Force in 43-44 had the highest casualty rate of any unit of that size during WW2.
THe Kriegsmarine U-Boat service was a death sentence. In the end, the U-boat fleet suffered extremely heavy casualties, losing 793 U-boats and about 28,000 submariners (a 75% casualty rate, the highest of all German forces during the war).