Do you mean the Atlantic/Pacific theaters?

I would definitely say Pacific as WWII America was far more involved with containing and beating back Japan than land battles in Europe for the majority of the war. But the Americans had it far worse in Europe as they were much less adept at European land war in comparison to the Pacific naval war.

The official US casualties were 185,924 deaths in the European/Atlantic theater and 106,207 deaths in Asia/Pacific theater. Going by casualties, the Atlantic had the highest casualties and by that definition was much deadlier.

Going by branches, the army by a mile and then some.

Of the 292,131 total American casualties in the second world war, the Army had 234,874.