After thinking more about this, I would have to say it would the US landing at Omaha. Casualties at the landing could've have been minimized if the planners had done a few things differently. Fortunately in the end, the landing was successful but at a heavy price.

Companies that landed at the "right" beaches were almost wiped out to a man. These were the units that landed right in front of the fortified draws.
But the units that landed in the "wrong" sectors barely suffered any casualties and made it off the beach in good order. They had accidently landed away from the fortified draws.

Also US Army planners in Europe disregarded the advice from veterans of amphibious landings in the Pacific Theater. The attitude was that Europe was the big league while the Pacific was the boondocks.